In higher tiers, Wolvers burrow when they are shot at. This makes for a horrifying experience because at times Wolvers burrow even while preparing an attack, often resulting in the unfair combo of appearing next to you and instantly attacking. Of course, this can be avoided by simply not firing at the enemies - but this means you cannot even shoot in their general direction without risking taking a hit, which renders guns almost useless in fighting a large group of Wolvers.

While this is not exactly game-breaking, not at least in comparison to Rocket Puppies (I'd rather fight eight Trojans than Rocket Puppies), this makes battling Wolvers frustrating because you instinctively expect to be able to fire at the enemy when they're standing still, preparing an attack. Removing this ability from Wolvers would not decrease their uniqueness and they would still be quite balanced, average enemies - it would merely make fighting them far more enjoyable.

EDIT: OH GOD THEY DO IT WHEN I TRY TO MELEE THEM