Their aggro is way too finicky and they shouldn't move around like they do.

I've stood right behind where they should spawn and hit them with a full combo'ed Butcher's Block, or really anything else usually besides an auto-attack as soon as they've spawned, and they still just take off for a healer or DPS immediately. They spawn, I smack them, I have top enmity, they run off, attack someone else, get to that person, then do a U-turn, and this usually takes 5-10 seconds of time I could use getting into a position or dealing damage.

They shouldn't be bolting for someone else if you hit them right as they spawn if you have top aggro.