The de facto greatest limiter to high end unit spam, already shown, is that producing T3 and T4 units will impose a Cosmite upkeep, thus inhibiting the production of further T3 and T4 units. Also, even the production of unit mods in general.
For example, if one were to have 5 Cosmite income by default(a common attribute in early images), then somehow obtaining an early T3 unit will cut that income down to 3, a 40% loss. Later in the game, perhaps you have 50, or even 100 Cosmite income. A stack of 1 T4 and 5 T3s would cost 14 Cosmite upkeep. Which is about 2/7ths of 50, or 1/7th of 100.
Ultimately, unless armies are significantly smaller, I foresee a lot of T2 units being used later in the game as well.
Now, ignoring unit tier variety, consider the following:
Each faction has:
2 T1 units (2 Core)
5 T2 units (2 Core, 3 Researched, 1 is naval)
4 T3 units (3 Researched, 1 is naval)
1 T4 unit (1 Researched)
(and 1 Colonizer, and often at least one special-summon unique unit of varying tiers, even if only in combat)
Compared to prior AoWs that means roughly two to three times the T2 diversity, and about three to four times the T3 diversity. Yes, one of each is a naval unit, but they are faction-unique naval units, at least.
Then the Secret Tech selection also adds a T2 unit, and either two T3 units, or a T3 and a T4 unit(unconfirmed if accessible by the Secret Tech, but a long time ago, there was implied to be a Xenoplague T4). Even if one wanted to directly compare Faction + Secret Tech to Race + Class, there are a greater set of units available to the player in Planetfall. In particular at the mid to high end tiers. Discounting the Archdruid and to a lesser degree Sorcerer's summon pool spells.
Then they are releasing with five Dwelling/NPC-Factions, each with a unit lineup of their own, with T1 through T4 units.
And Marauders, which can potentially be controlled, otherwise recruited, and in a few cases, produced.
Then lastly, as mentioned by others before me. Unit mods will allow diversity even within the exact same base unit, and this will be applicable to any unit you can get under your command, Faction/Secret Tech, NPC-Faction and even Marauders.
These mods will come not just from your Faction and Secret Tech selections, but also from your relations with NPC-Factions, and potentially to some undisclosed degree from Secondary Factions, as well as possibly even as special site reward unlocks(talking about not just the hero-unique mod rewards), or Landmark unlocks.