I didn't read through the whole thread, so it's likely I may repeat some stuff that's already said. I played through as Diana on Hard, romancing Galina and allying with the world, so comments are from that perspective. Will go back and try a few more in VN mode, but I wanted to get my thoughts down before I forget.
The card game was more interesting than I thought it would be. I think I enjoyed the gameplay here more than some of the RPGs like SotW. A lot of that had to do with the variety of the enemies - some cards were strong in some situations but weak in others, whereas aside from immunities, on the RPGs you can usually just combo a set sequence of spells. (I haven't tried QoT yet, so can't comment there.)
I think the card game was good enough that it's a shame Undead Lily won't be one.
That said, the difficulty was a little uneven. Most of the battles seemed too easy, especially 95% of the side missions. I gave up doing them after getting the achievement and didn't even use my last 25 stars or so. Most of the challenge there seemed to be cutting my deck down to 8 cards. I rarely had a resource issue, even with resource penalties from mission decisions, with the exception of mission #30 (more on that in a bit). Even on Hard, I was frequently sitting on piles of 5+ of the basic resources. Maybe the four resource system didn't pan out as intended?
The last mission was a substantial jump in difficulty - fitting for a final fight, but a bit of a surprising leap from missions prior. The difficulty came from having low resources and a ton of enemy kill abilities that just wiped out your units (the instant kill common/uncommon on summon is just mean - especially since half the bonus alien cards are uncommon/common). I actually had to reload and pick a different commander combo to beat this, but that's fine - it's the last boss. I actually think a few of the midpoint boss missions could've been more of a challenge - perhaps by being less generous with resources - but it's probably too late to bother changing now.
The missions themselves had a nice variety and I appreciated the ones that had special units or different win/lose conditions. I do think the special conditions should've been announced on turn 1 instead of turn 2, because often my team was giving me advice after it was too late and my regular strategy was underway. Also, later in the game, I think some of the special units should've been upgradeable. The one where you get common Dredans to help you had no effect because they all got instantly wiped out on the first enemy turn (the instant kill common/uncommon ability is a frequent cause, but also their low HP). And on the last mission, I wish I could've spent some stars to upgrade a few of the weaker units here and there because you're asking me to pick between a 2/3 Apex Rahn or a 4/3 gold Sharpshooter that both have the same effect.
But still, I did end up using all my deck slots, though I relied heavily on Diana/Galina with Diana/Mizuki near the end.
For the writing...
I liked the overall plot, especially the second half of the story. I did not guess the identity of the saboteur (though in the scene preceding the reveal, I began to suspect Alex was compromised or something). The races generally felt distinct (and the Palydrenian city was my favorite background piece). I don't think it was actually explained where the Descorians came from though? Were they advanced Robobrains? They just kind of...showed up, and none of the aliens seem like a likely culprit.
The friendly team talks made it seem more social than the other games. Instead of one-on-one, we got to see characters hang out with each other and explore their hobbies. It was also a nice touch to have a custom epilogue based on all the decisions you picked. I've only done Galina's romance path, so it's hard to comment on the romances - though that one in particular I felt was weaker than some of the friend story threads.
That said, I think there was a lot of excessive goofing off for a military team. Pretty much every major mission had some kind of diversion that was irrelevant to the actual plot and that threw off the sense of urgency. Like one of the later missions, you're gearing up to stave off a Descorian attack and there's a 20-line diversion into Rule 34 jokes. It was also kind of not helpful to see my subordinates openly insult the other races to their face, when some of them didn't even need to be in the room.
I think this is largely a result of trying to make sure all of the 16 characters got equal-ish screen time, but I think that was a mistake. Some characters didn't even really need to be there for certain scenes and some characters I weren't even sure why they were hanging around in the first place - Kaden didn't seem to contribute anything of value in particular. I think tightening down the character appearances and reducing the frequency of jokes would've made for a better plot/silliness ratio. A few strategically placed wisecracks are okay, and having looser tongues on the team talks is fine - and would make for a better distinction between serious we're-under-attack panic and relaxed free time.
I did like the invented history, references to things that happened in our future but the game's past, as they fleshed out the universe a bit more. I personally tend to cringe every time a futuristic game references pop culture in our current era since it feels anachronistic. We don't reference pop culture from the 1800s with regular frequency, so I'm not sure characters in a future time would make more than a couple passing references here and there to our contemporary history.
My comments on the character art can be found on numerous other threads, so I won't repeat them. Will say that I think the alien art actually turned out really well - the one place where I felt it was an improvement over the manga style of PS1. The backgrounds were also fairly neat as well (not sure but did you reuse a few from PS1? Like the bar?), with special props to the Maradan mushrooms and the crystal people's city. Also a thumbs up on the in-game music.
All in all, I think the game was a good addition to the PS line. I hope some of the setup and alien background lore will play a part in PS2 when that comes out next year.
I think all the interactions helped give the aliens more substance than they did as random mobs in PS1.
P.S. How long do Maradans live anyway? I thought Queen Schiller was in PS1?