Seraphim ACU is great and it's upgrades work well. It has numerous useful configurations, including an incredibly dangerous double nano + gun + splash combination.
Cybran ACU is great and its upgrades work well. It has fewer useful configurations, but is uniquely capable of telemazer and cloak/stealth mazer. Mazer is by far the best ACU weapon and is a beast.
Aeon ACU is alright but it's main weakness is that is has no strong, late-game weapon and no upgrades that greatly increase regen and health. It should be reworked as such:
Right arm 1: Speedx2 --[upgrades into]--> Range --[upgrades into]--> Speedx4 (total RoF increase with all upgrades = x8)
Right arm 2: RAS --[upgrades into]--> ARAS
Back 1: Chrono Dampener [Add small regen or hp bonus]
Back 2: Teleport
Back 3: Sensor suite
Left arm 1: Engineering --[upgrades into]-->Advanced engineering
Left arm 2: Shield --[upgrades into]--> Advanced shield
This will give Aeon a double strength gun and will allow them a shield/chrono combination to focus on combat. No nano regen will still be a disadvantage, but it's within acceptable diversity. Also, it will allow full gun and sensor suite, which is an effective combination.
UEF ACU is mostly good but needs a double strength gun, so it should get a Super Heavy Zypher Amplifier upgrade which increases damage from 200 to 800. (This is the only change I recommend as the UEF ACU is otherwise very good.)