With the empire you have a lot of options because they have so many different types.
I like to build an almost entirely mounted army with them as the horse archers can quickly flank and are decent archers while the cataphracts are super armored and can work like superior infantry when dismounted. It also goes with the lore of them having strong garrisons and being a mostly mobile army. The foot troops I find I end up putting into garrison. I think as long as you have the mounts its always better to upgrade them that way, if you wanted a bunch of legionaries and palatine guards just dismount the bucellari and cataphracts and you essentially have the same thing. I think cataphracts can beat up legionaries dismounted because they have more armor and higher skill, I don't know how the archers compare but they both have comparable skill levels and if you dismount the bucellari they shoot better.
I have also tried a Vaegir playthrough where every companion/family member is basically built like a sturgian version of a fian and I have a good archer captain and try to make the best Sturgian Veteran bowmen possible.
I also tried a Rhodok playthrough using green vlandians, not using the knights but just infantry and crossbowmen with good captains respectively.
Overall with the introduction of captains and banners you can make a lot of units workable that otherwise would not be, the only unit I really had trouble with was I was trying to do a Battanian knight playthrough and just use Battanian horsemen (unofficially the worst troop unit, not counting vlandian pikemen or swordsisters) but the main limitation I ran into was I needed to get the mellaurgy perk (+5 armor) and could not get engineering high enough in the vanilla game soon enough to really use them as well as really high medicine because I was trying to use them like cataphracts along with the 250 riding perk that also adds +5 armor
To be honest if you train medicine skill past 275 you can pretty much use any unit effectively, even sword sisters with about twice the normal hitpoints can be used and the ones that go down are likely to be wounded. Strat game showed you could do it with khuzait recruits and people have taken over calradia with just looters. The famous looton comes to mind, the Bob Ross of all looters, he is what every looter aspires to be and with your leadership you can help these plebs reach their full potential. Loot on looton, loot on
I play without mods because I feel like I cheat too much when I use mods but I have noticed that children get more attribute points so maybe make your first gen some time fast forwarding blacksmith/tactics or something or make them execute a lot of lords to get them mad and so you get executed and then you can play a heir, I think then you could really make a decent build without having to get to level 50 to get enough attribute points