Wait, how are Minoriko, Nitori and Iku among the "worst"? I've always found them immensely useful.
As the guy who made that video, perhaps I ought to explain my choices.
I started "Team Unappreciated" shortly after Ver. 3.01 came out as a sort of experiment to test out characters that I haven't really used before, and were generally regarded as mediocre. Meiling is the major exception; when I first started, I had claimed that I was confident I could beat the game with any team as long as I had Meiling and a healer of some sort. As for my other choices...
Minoriko - All right, so at the time I made that team, most people were starting to see how good she was already. However, it wasn't really that long ago that it seemed everyone liked Sanae more. Well, I always liked Minoriko (she's actually one of my favorite Touhou characters in general), so I wanted to show that she can keep up as main healer even without Reimu for support.
Rumia - She was definitely one of the least used characters, and probably still is. My main goal with her was to show that she can contribute more than just a good damage formula on Moonlight Ray. Dark Side of the Moon turned out to be a dud (it's still way too weak at pretty much every point of the game), but getting MAG buffs on Demarcation turned her into a competent group healer in the Plus Disk.
Wriggle - Poison is much-maligned, and that's not surprising given how difficult it is to measure. One of my goals with Team Unappreciated was to calculate just how much damage Wriggle was contributing with poison. Since I recorded all of the boss fights, I could easily go back and tally up the damage dealt, then subtract that from the boss's HP to find poison damage. I concluded that Wriggle's poison is quite powerful in the main game and definitely worth a party slot, but gets severely outclassed in the Plus Disk due to everyone being faster (thus giving less time for poison to work). She's still a great 2nd slot subtank, though.
Iku - She's the discovery that I'm most proud of. Before I started Team Unappreciated, I don't think I knew
anyone who used her seriously on their main team. Her wiki entry was full of wrong information, describing her defenses as "average'" (lol best MND tank in the game) and listing Thundercloud Stickleback as giving only +50% ATK/MAG. I think the earlier tier lists I read listed her as low tier, or thereabouts. Even I myself bought into that. I'll confess that when I was first drafting up Team Unappreciated, I left Iku off the list because I had no clue how I was suppose to use her (my goal was to let each character shine in some way). Someone else had to convince me to give her a try. Soon, it hit me that Thundercloud Stickleback was actually giving +72% despite its in-game description. Then I discovered her monstrous MND growth (after wasting 60+ lvl bonuses on MAG). Eventually, my perspective changed from her being low tier to wondering how I ever played this game without her.
Nitori - When I started up the team, I had a hunch that she was going to be good. However, remember that I started the team right after 3.01 came out. Back in the 2.04 days, Nitori was hardly ever used by anyone. That's because the old damage formula for Megawatt Linear Gun wasn't that amazing. Well, it was pretty good, but Nitori's subpar stats really hurt its performance. After switching to 3.01, though, the damage became incredible. I knew it had been buffed, but I didn't realize just how big a difference it was. It didn't take too long after 3.01's release for people to figure out that Nitori is a top tier damage dealer now, but when I first made this team most people weren't sure just how much better she had gotten.
Reisen - The other discovery that I'm proud of. Reisen was considered by many to be the absolute worst character in the game. You should have seen her old wiki entry; it all but outright said so! And of course, I believed it too. But then I stumbled upon a detail that was missing from her wiki data page: Grand Patriot's Elixir has a post-use gauge of 84%. That's absolutely INSANE for such a strong buff! Once she had enough SP to spam it during boss fights, she easily became an awesome damage dealer that could also debuff well. I didn't even care about the debuffs in most of the Plus Disk fights; just her DPS with Mind Starmine was enough. In the Plus Disk, she was second only to Nitori in terms of damage output. She's still one of my pet favorites in the game now.
Mokou - She's a really "average" mage-type character, and I'll admit that I didn't really show her to be fantastic or anything. I don't think people thought she was "bad" per se, just really boring and typical. She trades some damage for survivability and is not limited to a single element like many other mages are, but I think I failed to show her off adequately, especially compared to the next character. Sometimes I wonder if she'd appear to be better if I didn't use the next character. Still, her survivability was important for me, as I often switched her in to take a hit in place of Nitori (who pretty much never got hit throughout almost the entire Plus Disk; I'm not risking my best weapon!).
Orin - At the time, popular opinion was that Orin was great at clearing floor trash, but poor against bosses. I set out to disprove this notion, and I'd like to think that I've at least gotten people to reconsider. Blazing Wheel really is a great nuke for most boss fights. It has the most powerful damage formula of any composite spell in the game, and Orin's stats are just good enough to make use of it. Her high speed is also key, as it lets her spam Blazing Wheel (which doesn't really have terrible delay in the first place) while staying in. I even found a niche use for Cat's Walk (see my Flame Tyrant video). She can usually take a hit from magic thanks to her high MND growth, and to be honest, Blazing Wheel was slightly outdamaging Fujiyama Volcano in almost every battle. I suppose she stole some of Mokou's thunder...I'll admit that I like her more thank Mokou.
Keine - General consensus was that Keine is average in every sense of the word. And after taking her all the way through 30 floors...I'd have to agree. She never stood out aside from above-average durability, and her ability to buff an entire party's offenses was kind of overshadowed by how broken Thundercloud Stickleback is. And yet, she never disappointed me either. She fulfilled her duty reliably, but didn't go above and beyond any expectations. All I have to say is that I wouldn't be unwilling to have to use her for offensive buffs instead of Iku or Ran, but I'd still rather have Iku. I bet if I didn't have Iku on my team, she'd shine a lot more.
Maribel - Everyone was saying that she's probably OK on paper, but no one ever seems to talk about actually trying her out. So that's exactly what I did. To be honest, I found her to be a little worse than Reisen, which is by no means an insult, as I thought Reisen was great. If I didn't have Reisen on the team, I'd probably have appreciated her a little more. As it stands, I think all I ended up demonstrating is that she's perfectly usable, but not particularly exciting. Oh well. I liked her spread of elements, at least.
Renko - Another "good on paper" character, but actual stories of people using her were hard to find. Some people were probably put off by her non-existant damage output. Well, I thought Cirno wasn't that bad for the main story during my earlier Let's Play, so I tried her out...and found out exactly why she's normally only recruited when you're halfway through the Plus Disk. Galaxy Stop and Debilitate are both incredible, and completely shatters the difficulty curve of the main game. In fact, I had to stop using her until I got to the 20F boss because she just made things too easy. Her usefulness in the Plus Disk started to deminish near the very end, though, as the 30F bosses boast some pretty high resistances to PAR and debuffs. To be honest, I didn't think Charge was that great in practice (only really used it for maybe 2 boss fights), but her other abilities still made her worth it against the vast majority of bosses. Just keep in mind that there are a few that she's nearly useless for, so it might be better to leave her as a substituted and switch her in for certain boss fights where she'll clean house.
Well...that went on a lot longer than I expected. But those were my thoughts concerning Team Unappreciated.