Are there any disadvantages to using earphones instead of headphones while practicing guitar quietly. Is the sound quality that bad ?
Earphones are fine for practicing. You may not get to wallow in the full sonic wonderfulness of the instrument and amp. So what? They also discourage your neighbours from getting homicidal towards you. This is an enormous and overriding advantage.
If you're practicing technique, scales etc. maybe you don't need amplification at all?
The quality of sound will always depend on the gear you are using. A cheap $10 pair of earphones will work if you just need something to keep quiet. There is an issue though, you'll need to by a pocket amp (available on Amazon for $25+). Most guitars don't have any amplification built in (in-fact, I don't think I've ever seen one). Your traditional earphones/headphones have a smaller jack than a guitar cable.