In the case of zombys I believe when you get more than 10 all your pets vanish. In the case of mustache golems when you summons one it will often make all your pets vanish or make your slime vanish. The same has the same case as the mustache golem. Pretty sure only supposed to be able to have one mustache golem but it shouldnt case other pets to despawn and the same goes for the smile. I have a feeling other pets behave similarly but have not tested it.
Not a bug, but a feature, I believe. It's so that you can't have a small army out all at once. Things like trained hunting diggles or zombys are relatively weak, so you're allowed to have more than one out at once.
Found another pet oddity. If you leave a pet behind on a DL, monsters on the current level you're on can still swing and damage it. It can also kill the monsters. It's my Invisible Ninja pet!
So wait a summoner build isn't supposed to be viable in the slightest? That's really lame. I can understand a limit of say 5 zomby's, 1 mustache golem, one slime, one etc of the stronger summons but having other pets be unsummoned with no warning is bad. So it's either a bug or a poorly designed feature.
Also, Pets are not being derezzed upon creating a new one - if you're on a different DL and left the other one on another. Could be a mis/feature
"Too good?". Eh, they're near useless (not completely, but almost) at anything DL5 or lower. Wish at least the last pet in a class would scale, m'self. Another thing is, why are my pets so d-u-m-b? I mean, they get stuck on tables, around corners, on vending machines . . . . . the monsters after me, however, track me with the efficiency of a bloodhound.
I find the early dungeon levels the most difficult ones. By the time you hit DL5 your character is usually pretty well set up. I agree that the lack of scaling is odd.