For the most part, T1 units get annihilated by their T2 counterparts and so on for T3 vs T2, so they're largely upgrades. This is a simplification, but the point remains that in SupCom you advance in technology by unlocking new, more powerful and visually distinct units that eventually form the bulk of your army, replacing earlier units - with exceptions.
I haven't played much AotS yet but I was under the impression that the dreadnought units, with their very long build times and limited resources on the map, were essentially complementary to the early factory units rather than a technological upgrade over them. In any case, with infinite upgrades, basic factory units could easily become more powerful than opposing dreadnoughts.
For having played both games intensely (as intensely as you can have played AOTS thus far) I can tell you that few people here realize how much more complementary different tiered units in FA were than in TA or AOTS.
here is a short example set of units that are the ONLY ONE all four tiers and all four races combined to have an anbility :
- Cybran t2 mobile stealth : the Deceiver. only mobile steath field unit in the game (it's stealth remain active in transports even if loaded in a different race transport and even if loaded with different race units. it also is the only unit in the game than can be loaded into a transport slot if the transport is already full)
- Cybran t2 :Fire beetle. only land kamikaze unit in the game.
- Cybran t2 Destroyer : Salem. only naval unit in the game to be able to traverse land
- Cybran t2 Cruiser : only naval unit in the game able to fire it's air guns on ground targets
- Aeon Mercy : only kamikaze air unit in the game
- UEF t2 shield boat : the Bulwark : only naval shield unit in the game
- UEF t3 Novax : single orbital unit in the game (this unit has an entire layer to itself)
- UEF commander only commander to have a strat nuke backpack
- Cybran commander only one to have torpedoes and the monkeylord lazer
- Aeon com only one to have EMP
- seraphim only one to have augmented regen and regen field
if you look into structures the diversity becomes much stronger;
- only one t3 torpedo defense in the game
- only one exprimental resource gen
- only one map scan structure (the ui for this exists for only this unit)
these are two very shortened lists but even within the same race from one tier to another you're not finding the same thing.
mobile shields for example don't have an upgraded version. If you're UEF and t3 you have tap into your tier two to accommodate your t3 troops with shield coverage. for Seraphim it'll be the opposite : their shield is in the t3.
-"why would you wanna send t2 at the t3 stage!" Seraphim t2 bot is more adapted to a good percentage of combat scenarios than their t3 tank is whereas sometimes it's the t3 tank that is more adapted.
all t4 units have clear distinguishing traits with unit's whose functions you might find comparable : the fatboy for instance is not the same thing as "a tank, just stronger". it features range comparable with no other land unit and is a factory and a air repair and refueling facility.
mobile artillery only sees an appearance at t1. mobile missile launchers aren't an upgrade to it their projectiles are too slow and do less damage -"wait, what; less?!" yes, less. Their incredible range makes the mobile missile launcher the ideal shield+ t2 +t1 defense turret breaker despite it's low dps.
since the zhuee hovers it remains useful all the way to the last tier of the game because they can apply serious damage to the good targets that are slow moving t3 naval or simply undefended bases by taking shortcuts through water.
also tier three in UEF sees the addition of both a t3 mobile missile launcher and an artillery. which have vastly different projectile velocity, behavior and damage. which one's the upgrade now ?
starting t2 you almost always have the choice between two (or three!) different land units with a wide variety of characteristics and abilities. it's never going to be the same ones you chose to use primarily according to the situation as well as according to your current tier.
for example : t2 hover tanks synergize strongly with t2 (but mostly t3) navy. the UEF t2 bot, on the other hand becomes a good partner with the t3 percival.
In all races at tier 3 you'll want to accompany your t3 army with t2 anti air for it is the best anti air against gunships. t3 mobile anti air (yes we have that, that's a faf unit) will be less efficient versus gunships and more against t3 bombers.
when any other race then aeon and using air to attack navy you'll use t2 torp bombers assisted by your t3 fighters.
if you are cybran you'll often prefer t1 gunships over their t2 or t3 (dare i say "and even t4"?) counterparts if you are in a situation of being interested in sheer damage rather than speed
and in a similar situation as seraphim, your t1 bombers might weigh more favorably in comparison to your t2, t3, or even t4.
you built all those undertiered units throughout the game and i'm missing a bunch of examples.
not only that but contrary to AOTS. FA and TA did not feature armor mechanics determining if lower tiered units do lower damage to higher tiered units or not.
I hope I hammered the nail in deep and we move onward in life knowing that TA and AOTS have got nothing (so far! (i'd like for that to change for AOTS)) on FA in term of cross-tier unit compatibility and that tiers in FA aren't "upgrades" contrary to urban legend.