With my Shield that has the small manual safety, I'm using the technique of putting the safety on, carefully holstering, and then taking the safety back off. You can do that with the partial sweatguard but not the full. I personally find the full guard a tiny bit more comfortable (and have it on my G19 holster) but not enough to really make much difference. I don't have a scenario where I am wearing the pistol directly against my body in a hot and sweaty environment; so I can't comment on that.
I don't personally find any disadvantages to the full other than the safety thing I just mentioned.