I love using bad units, or rather units who people perceive as bad, because the vast majority of the time they are much better than you may think. FE6 Barth is a good example of this. (no stat boosters, pretty average apart from +1 or 2 average SPD, level 20/3 or so)
Having interesting growths or a cool stat/skill build is another point. I really like Darros and Astram in FE11 because of their lopsided Defence and STR growths. So if you make Darros a Knight he can easily cap his DEF, while Warrior Astram will actually become the strongest man in Archanea.
They can also just plain look weird, cute or have a personality i like. Norton from TRS, aswell as Nephenee and Echidna are a good example for me.
Having something unique about them is another good indicatior. Tom from Tear Ring Saga comes to mind because, well, he’s a literal tank, no not an armor knight, no not tanky, but a literal WW1 style tank. That alone makes me wanna use him.
Lastly, if you’re ironmanning, as i always do, you can sometimes just be forced to use units you normally wouldn’t, It’s always fun if they suddenly turn out better than expected, or literally save your run, like Curate>Paladin Vyland and Hunter>Warrior>Horseman Macellan did for me. Stories like these personally make using the units again a lot more fun.