Questions about how the meta has changed since GPG days

From my memory restorers were rarely used cause of gentleman agreement. Sometimes loosing side player may start to build restorers... then winning side on setons also started making restorers xD. But yea it was a huge problem.

t1 aa got cheaper but lower HP, t2 aa got some buffs with minor faction diversity. It is no longer a big mistake to use it instead of mobile flack. In gpg people used static t2 aa only becasue if they used mobile one they could accidentally move it away.

t1 pd... i i think there was no changes. Some changes were made to t1 arty. UEF has less frontloaded damage and usually takes more time now to kill a PD. Cybran also less damage, less splash, but more fire rate i think... i think cybran arty should be tiny bit better against PDs compared to GPG.
BIG change to PD vs t1 arty is that if you have radar then you can simply attack move with arty, no need to manually target PD.
Thats why almost no one makes fake PDs right now

T2 arty once had a 25% cost decrese and it used to be pretty cancerous in teamgames and people build that alot. t2 arty is a counter to fatboy. not sure how exactly it works now but people still build it sometimes.

wagner was nerfed alot, but frigate sonar was also nerfed alot, so it became little bit stealthyer (still can't hide from air scouts or torp bombers) but wagner has really low HP, like 1300 or so, i don't remember. while rhino has 1900 hp. Rhino is still hits ground like in gpg and still misses units but i think less miss compared to GPG. but not 100% sure.
T2 bots had alot of balance changes back and forward, also mobile shields...

Cybran engineering stations are still good. Can't say much good about UEF engineering stations, they are usable, they can be used to steal mass but they are far inferior compared to hives in late game. They don't react instantly like hives and they die to enemy arty shells.

I'd say don't overbuild power storage early on. Fire beetles still don't seem to be very usefull, but maybe nobody tryed hard enought. only 1100 damage now, but has cloak (but no stealth). Wagner is pretty bad. Don't make corsairs for air superiority. Don't make restorers or continentals for air superiority.

i think your questions do not cover that many things... there was so so so many changes... tho you can still spam t1 then transition to higher tech. There is now less reasons to rush t2 or rush t3 or rush t2 or t3 air. also labs and selens were buffed so "lab phase" has also been slightly extended

The biggest changes I could list would be Engie mod, T3 rebalance, Veterancy system rework, OC rework.
Answering your questions:

Static t1 AA is more efficient then mobile one but as name suggest it can't move. T2 static flak is meh and you rarely build it.

T1 pd surrounded by walls is really strong and unless enemy has arty in their unit mix it's a no go zone for t1 tanks.

T2 static arty is very expensive but it can be useful, mostly in more turtly games or built as a response and counter to a Fatboy.

Rhinos were buffed and they are a good t2 unit now, Wagners were nerfed and they are a raid unit only.

Engineering stations are a lot less efficient then engineers and they are only really built in late game.

Most of the units have a specific role and most of them do well at it.

Thanks @tagada and @zlo, very helpful.

UEF T1 mobile arty is particularly interesting. I think I recall that being 600 alpha damage in the old days. Looking at the database now and it's a little confusing. It says 100dmg (60.24dps) vs the Cybran one at 230dmg (38.33dps). And the UEF one shoot 5 shots/8.3 secs, so from that I guess that instead of doing one big starburst shot it does five smaller ones?

Does the Cybran one still stun, and can that stun affect air like it used to?

They buffed T2 Tanks to be more equal and less broken, nerfed hover speed a bit, buffed the heck out of the Blaze so that it isn't a litteral meme anymore and reduced alpha damage on Obsidian and Percival in exchange for higher fire rate (still strong alpha weapons).
Specter T2 Aeon Gunship got its weapon changed to fire a shotgun of oblivion weapon, meaning more alpha damage ; CZAR got a shield, which is funny.

Overall T3 nerf to try to extend the T2 phase if I'm not mistaken. Experimental build time nerf. sACU rework coming soon to try to bring in factional diversity combat support sACU to help the T3 army get some relevance.

T3 Mass fabs are more viable than GPG.

T3 Mobile Anti Air are a thing, they're cool but nobody uses them.

You don't see engineering stations as much as a direct result of engie mod. Making more factories is a much more efficient way of increasing build power now.

I made a recap of the differences on arqade, dunno how helpful it is : https://gaming.stackexchange.com/questions/130147/how-does-a-forged-alliance-player-learn-to-play-fa-forever/353014#353014

@tatsu that's really helpful, thank you very much!

@Wainan you're welcome!

@tatsu I would venture to say, after reviewing a team game with Yudi 2-3 days ago where he goes first Strat on some sort of coastal map (might've been a 5v5 opti iirc) that hover bombing is still a thing. And he pulls it off effortlessly too! Don't have replay Id as away from PC.

Back in the day you could stop a bomber mid flight and slowly move backwards by clicking behind the bomber. The bomb cooldown was also shorter. This allowed you to eg. land 3 bombs on pgens in a quick succession. Nowadays you can't hover backwards with t1 bombers anymore (well you can but it breaks after a few meters and is super slow, also takes some skill to pull off) and it's pretty limited with t3 bombers too. With the higher attack cooldown it's better to do short passes anyways.

Yes you can still micro the bomber, you just can't* go backwards anymore. The usual use case for micro is to stop bomber mid air to let it reload so that you can get multiple bombs off in a single pass (but not against a single target like before)