It's not that I don't like Haste, it's just that I don't love extremely slow actives on my spike teams. I don't know what other NepDra teams look like, but mine has way too few skillboosts relative to how painfully slow NepDra's active is. I'd rather have moderately fast actives with moderate haste than very slow actives with a lot of haste.
I'll give you that one poison is occasionally extremely useful, but two poisons is almost always unnecessary. Sure, one is necessary for some dungeons. But Neptune is forced to run two and has four slots for his remaining actives, even if he does let them be up more often because of Haste. Quite a few of those four slots will need to be orbchanges. But since Pandora is an orbchange she can stock a poison and two more orbchanges and she'll already have as many as NepDra can have even if he forgoes all other utility effects with room for a shield or a bind clear or something.
It's also possibly that my NepDra team is just trash. Maybe if I had higher-HP subs I wouldn't have such a fixation on wanting more HP. But the way I see it Neptune Dragon has three major problems: Insufficient HP, too slow for lack of skillboosts and a slow active, and is one of the shit colors and therefore can be resisted. Pandora has none of those problems. I mean, I'm going to try her when she comes out, I do have one (although I don't have anything close to perfect subs for her) so I can see for myself where she falls short. But right now NepDra looks like an almost-perfect team with a few crippling shortcomings and she seems to have none of them.
Pandora is unresistable until you remember that GungHo loves to put in enemies with Dark absorb because FUCK YOUJ
Also like Suikama mentions, GungHo also love to shove in Predras into fucking everything INCLUDING Arena 2.0 so having to either waste a spot for a poison/true damage or praying to RNG god your Grisar can do 10M+ damage on burst w/ A.Loki is not always sound
It's not like Nepdra has bad RCV or HP either

Keep in mind that Gungho also likes to put color resist out for things that are naturally resistable anyway (Awoken Shiva). Sure a handful of things artificially resist black and white, relevant things even, but 20% of the game naturally resist blue, quite a few of which are relevant, with some extra "fuck you"s on top of that.
As for subs, I guess I can't really comment on optimal teams for either of them. I own literally zero of those subs. Maybe NepDra's weaknesses can be covered with an absolutely perfect team, but Pandora doesn't need that to have solid stats and presumably has even better stats once she does.