Do you mean ones like this:
Certainly the most common and available design.
These can be a pain to fit (not difficult, just a pain). Depends how tight the ring is. You can also see how they are a bucket design and will trap muddy water which will rot the diff case. The recess is not perfectly aligned with the fill plug and does make it harder to refill as you can't pour from all angles when filling.
They also reduce clearance by the thickness of the guard plus a bit. It might not look enough, but if the diff would be touching without the guard, it can and is enough to stop a vehicle.
And incidentally I have seen these fall off. Occasionally people have attended trials with these. And while some are fine, I have see a few end up laying on the ground after a person has driven through the terrain.
I'm not saying they are 'bad'. Just arguably 'worthless' on a D90 off roading in the UK. And the only place I ever hear of people damaging diff casings is on the internet. Never once have I witnessed it in person.
