They can't be gelded, that would ruin the fun
You can generally keep them under control as warbeasts if you make sure they don't outnumber the squad members. Assigning several to accompany a single hunter is usually a bad idea, though.
They are a little less volatile this time around - I had to make a whole new script to deal with the new AI for interactions. (To put it simply, creatures interact a LOT less frequently now, and more randomly at that, so the population counting system is less reliable overall.) Less volitile, but also less predictable.
To aid with this somewhat, I left in a few of the indicators I used for testing, PHM thinking about rebellion will blink with a purple question mark occasionally, and when they go into rebel mode they will flash with an exclamation mark. Not sure if people will prefer it this way or the old way. It fits the mushroom-men vibe a little better when you could never tell what they were planning until they suddenly attacked someone. On the other hand, it's fun to watch the exclamation marks spread among the population. (Not every "revolt" will actually lead to an actual attack. There's a lot up to chance.)
It's pretty rare for PHM to actually gang-rush a target right now; they have to outnumber non-PHM by five to one before they'll go all-out. They might occasionally take a swing at a passing dwarf at random when revolting though, which can potentially lead to all kinds of chaos. Actually, most of the damage caused by a revolt seems to be from dwarf-on-dwarf combat in the aftermath of the revolt itself. Loyalty cascades don't happen in DF2014 in the same insane way they used to, but it seems that if a dwarf sees another dwarf chasing a PHM through the halls, not realizing that the PHM punched him first, he will get angry and start trying to defend the 'innocent' mushroom man by attacking the 'aggressor'. Usually there are only a few fistfights before it dies down but if you've got some hotheaded miner swinging his pick around during a brawl, accidents will happen... I've also seen one instance of a farmer going all Captain Ahab on the particular PHM who broke his leg. He kept dragging himself after the PHM and getting beaten up by the other dwarves around him. Everyone was feeling 'vengeful'.