I have roostley and bai maxed and emblemed.
Roostley is good for tile damage and defense. Use him as a softener and finisher of low health heros.
Bai is most suited for offense. He is very useful similar to Malosi, his perk is that i can now avoid bringing a healer to purple titans because coupled with joon, titan has blind/miss all the time. Dont forget his elemental link is great as well.
Mica, dont have, i see not much use from him to passive for my taste. though, i hate the ninja family bonus and since you have cobalt they would pair nicely on defense. As yellow lacks good healers him being able to reduce incoming damage very early on, could be helpful, if you have survivability issues.
Out of those 3 you will have i think the most fun with Bai Yeong. His blind may fail some times, but think of probability more like this:
His blind with 75% miss chance is comparable to the chance of pulling 3* hero from portals (nsead of 4* & 5* heros = miss fails) and we pull 3* heros all the ■■■■■■■ time.