Probably the same things everyone else who made commentary on the subject did; difficulty spikes
Difficulty is pretty steady in infinity, it gets progressively harder. You just aren't allowed to die or your whole run ends (which can get kinda sour, I did great up till level 14 when I had an accidental run in with a torpedo). The later levels especially get balls hard.
poor balance in general
There are some issues when it comes to reinforcements. Interceptors and Fighters are a complete waste of credits for example. You'll be in luck if they ever get a kill in and I'm unsure on how the devs didn't notice how much better a frigate is in the anti fighter roll. There's also that you get half the credits back at the end of the mission for every surviving ally. Your fighters are very likely
not going to be alive at the end of the mission.
There is no way to tell your allies what to do (afaik), so when you've got fighters there will be like 2 attacking an enemy cruiser while 3 are someplace else fighting a corvette while your bombers are slowly getting killed by enemy fighters. This is also what makes bombers useless, you can't call them in when you need them, they'll be there at the start of the mission and half of them will probably be dead by the time the first capital ships show up.
overlapping controls for Strike and Pursuit modes that don't do the same things with the same keys.
I don't get how this is a complaint. I have absolutely no issue with this and I think that the people that complain about this are just dumb and bad and terrible (people and gamers)
To that I'd add that Strike mode in general is pretty disappointing and the only functionality of worth it appears to offer is the multi-lock missiles for coping with inbound torpedo salvos or warship turrets. Otherwise its penchant for absorbing more fire than Pursuit mode tends to outweigh any benefits.
Strike mode is actually prrretty amazing. If you are absorbing more fire in strike suit mode than in pur suit mode (teehee),
you are doing it wrong. Strike mode gets so much extra mobility, dashing back forth left and right allows you to pretty much avoid 90% of the enemy fire. It's turning rate is also incredible, I often use it to just achieve a quick 180. Make a strafing run on a capital ship, afterburn away, transform, twirl around, transform again, strrrrrrafe like you wanna win.
I'm not completely sold on the Flux mechanic but I've long time ago come to terms with Flux management as part of the game.
Speaking of strike mode, Infi has two more strike suits that quite change the way you can play. There's the fat and heavy marauder which has incredibly powerful long range cannons and (unforunately pretty useless) multi lock mini torpedoes. These cannon's have a long reload, absorb a good bit of flux when fired but can pretty much eliminate a corvette in like 2-3 hits at a range of 7000. Not as great against hordes of fighters since you use more flux per shot than you get back from destroying an enemy fighter with it but if you have a full gauge you can still safely snipe away the majority of them (1 shot 1 kill)
And then there's that other strike suit which is fast, less armored and has a shotgun as primary. That's all I can say about it really since I haven't really used it much. You'll be cruising the battlefield quite a lot so I prefer the long range weaponry.
Also the multi-stage escort missions do not appear to reference other stages for the condition of the ship being escorted.
Well rejoice, there are no escort missions in Infi (though if you have capitalships on the field you'll spend quite a bit of time defending them)
And it makes me amazingly mad, too, because a lot of this game is a combination of all that was good and fun from Freelancer and FreeSpace, and then it goes off the rails into frustrating bull****.
Well if you really found SSZ that incredibly frustrating then you might still not like Infi either. Despite its flaws I had quite a bit of fun with SSZ.