I haven’t found a disadvantage yet
1. Low force required to cause failure.
2. 10x more expensive than the alternative
3. Single use device
4. Metal on metal connection can be noisy if loaded and unloaded.
5. Another chunk of metal to make noise if you bump it.
6. IT IS NOT DESIGNED TO BE USED FOR FALL ARREST
And don't try tell me there is never slack in the line so you won't fall on it. You can't climb a tree without ever introducing slack into the system unless you are ascending the rope, which is the only correct use for an ascender. I don't think you guys realize how much force can hit each element of your safety system when you fall even a foot or two on static rope. This is even more true with the ultra low-stretch rope everyone is obsessed with around here.
You can adjust a ropeman with one hand both ways. A prusik takes two hands to tighten, or at least some nifty fingerwork. I use a pusik on my tag end of my tether for backup. Also for backup if I repel.
Yes, I think we've covered the one and only benefit. I should point out that I rock climb and am VERY familiar with all the gear we discuss. I love ascenders... for ascending rope. Try a friction hitch tender if you need one hand adjustment.
Another point., a friction hitch isn't the best backup since they need to be dressed and snugged up to work properly. You can't beat a direct tie-in to the tether/lineman to back up your adjustable connection, whether it be friction hitch or ascender.
As long as you are backing your stuff up, you can probably get away with misusing an ascender for this purpose and not die but let's not pretend it's the best or safest solution.