Most units in the $100+ range won't hurt your signal. Go any cheaper, and your quality will fail. I've never noticed a relevant audible difference between line and wireless on decent units.
I've never heard of a reason not to use it for electric and acoustic; it's whole purpose is to wirelessly transmit a perfect reproduction of the signal, so it shouldn't matter what you use with it. Theoretically I suppose you could use it with a keyboard, drum machine, or just about anything else (as long as the levels are all good).
The main worry with wireless is battery life. Those things eat through batteries, so make sure to load it fresh right before you go on, and turn it off whenever it's not in use. Make sure if other people are using wireless stuff that it's on different channels or you'll get some interference (bonus points if you get a wireless that lets you pick your channel). Also, if you're doing this to increase the amount of distance you can get away from your amp, keep in mind that you're still going to need a long cable to go from your wireless unit to your stomp boxes (assuming you use some). Most good units will give you at least a good 50 feet of range, and most will give you far over 100 on a good battery.
Anyway, I haven't used these things way too much, so please, somebody correct me if I'm wrong about any of the stuff I mentioned