I actually kinda had fun with NuArena this last week because it was something different and forced me to play differently than usual, but I'm definitely not going to claim it's good or balanced. A lot of things are basically auto-scoop indicators, including one map, which is pretty silly.
The one thing I can say for the actual strategic depth is that there's a lot of merit in using your non-bonus units for combat if you do it very carefully. Baiting ranged units so they can be engaged one at a time or bringing Special Fighter users with a color advantage against your bonus unit down to low health can take a lot of planning and strategy. But it's also only remotely viable part of the time. Maybe I could have built the sets on my support units in a way that made it a bit easier, but as it is the stars kind of need to align before you can even think about fighting with your support units.
I think the best case scenario is that the arena rules get shaken up on a semi-regular basis to force different playstyles. Right now we have "tentpole meta", maybe in a few months we could have an arena metagame where you get a bonus for getting one kill with each of your units and not doubling up at all or something. I know that's probably not going to happen, but it's the best solution I can practically come up with.
What it all comes down to for me honestly is that I don't care that much about Arena right now, especially since I got my crown. With Abyss maps being a thing and with the promise of many more on the way it's no longer the main lategame content. I know not everyone loves Abyss, but I do, it couldn't be more fun for me the way it's balanced right now. And I get a lot of time out of it, each map so far except Hector has taken me close to, if not more than, four hours to crack. That's kind of an unproductive thing to say, but while the arena is almost unarguably broken I'm pretty happy that I have another big goal to work on.
Grand Conquests feel sort of similar in that the top tier difficulty setting is genuinely hard content age is different enough from the normal game to encourage unit diversification and not just be coasted through with one core team, but the rewards for Grand Conquest are pretty poor compared to pointless cosmetic trophies.
Edit: Oh hey the new Abyss maps are out. Being on a 24-hour rotation kind of isn't great, these usually take me much longer than that to work out or even get to since I don't have time to play them most days. But we know they'll come back around pretty soon so I guess it's fine.