speaking from experience, navy feels (by far) the most balanced thing in the game
Navy also has the smallest numbers of units:
- T1 land? Scout, (lab), tank, AA, arty
- T1 air? Scout, inti, bomber, transport
- T1 navy? Frigate, sub (plus Aeon AA boat)
The frigate is simultaneously a scout, front-line direct-fire unit, anti-defence unit (beating T1 PD on range at the same cost and trading decently vs T1 torp launcher) and (except Aeon) AA unit.
Sure, the frigate costs significantly more than most T1 land or air units but somehow it is a single unit able, mass-for-mass, to beat any T1 land unit or structure, when in range of water (range means only T1 arty can fire back). If T1 PD were able to match frigates on range then at least there would be some counter.
Probably a lot of the land/navy imbalance is due to forcing navy to have a role in a game where almost all base-building is restricted to land and confined enough to where there is only 1-2 significant bases worth defending. That said, navy is (IMO) a lot more fun on maps like Ndera where frigates have many raiding options than maps like Seton's Clutch.
So, can frigates be balanced well in both the role of T1 tank (front line) and T1 arty (base busting & raiding)? Kinda.
Then there's T1 subs... subs only beat frigates if the frigate's mission is to destroy the sub. It's not. The frigate completes its mission (destroy engies/mex/navy factory) before the sub kills the frigate. Effectively the sub loses, even if it lives and the frigate dies.
IMO to make subs useful, they should become ambush hunters: able to lurk stealthily (maybe requires cloak as above) and having a high-alpha strike (high initial damage, long reload time. E.g if T1 subs fire a torp salvo with total 750 damage and 20sec reload time, DPS is unchanged but 3 subs can kill 1 frig instantly. I'm not convinced that even this is enough to make T1 subs actually useful besides mex hunting.