Yeah I'm still not clear on what degree affixes can be stacked without being displayed / if any get dropped. And in some cases, like TZ cursed with a cursed unique they display and don't work. Needs some more testing.
There are a few demon superuniques that always spawn with 2 affixes that aren't extra fast / extra strong / aura enchanted / spectral hit. Namely pitspawn fouldog (cursed+cold), Toorc Icefist (cold + stone skin) and Wyand Voidbringer (mana burn + teleportation), thus if they randomly roll affixes other than the 4 given by bind, they should have 8 total affixes in hell when bound. Those would be the monsters to test to see if affixes get dropped.
Some other things I saw is that those different "Blunderbore" variants seem to be having vastly different speeds and I can't get a read on what's causing this. My first impression was that they were so much slower than Hephasto, but now I'm not sure. Of course there are many other things to consider to decide a better demon, but their movement speed at the least seems to be slower.
I also managed to bug out some of their mob's minions to a point that they would follow me and my allies and attack but not deal any damage and nothing I did could even touch them. Some skill swapping and killing other mobs solved this sooner or later.
I could even hold down Bind Demon to rapidly deplete my mana, not catch the demon and somehow it also randomly replenished like half my mana and then drained it again as I held the spell. Weird.
One other thing I wanted to ask is have you tried maybe doing a build that specializes on boss killing, as those summoned Goatmen have Crushing Blow? You could have a bound demon Urdar with two goats all dealing CB.
That would of course require another build as the goat CB requires Death Mark points or it's probably just not worth it.