I mean, troll warlord already loses control over his hero, and his ultimate has a pretty long cooldown too.
It's quite sad to see him spend his ultimate chasing an illusion or hitting a wraith king aghanim scepter ghost. Especially bad when the illusions are terrorblade illusions and everyone can see they're fake. Or vengeful spirit aghanim scepter illusion.
I know you can "lock on" to one hero before your ult starts. But troll will reacquire a new target if you change to melee form, or if that target disappears for 0.1 second (e.g. casting manta style makes a hero disappear briefly and troll will find a new target), or if that target moves 700 range away etc, or if the target casts eul's or goes invis etc the list goes on.
I don't think it'd be too strong for troll to simply ignore illusions in that precious 6.5 seconds would it, considering you don't even get to control your hero in that duration