What are the drawbacks of an automatic card shuffler?
Mechanical shufflers come in several types. The simplest are the hand cranked gravity fed multi-wheel versions... and are the only ones I've much experience with, having owned two. (Both inherited.)

These particular ones both use a wheel to throw cards into the central bay from two elevated bays.


some cards end up face up - I've averaged 1 face-up per 3 shuffles or so.
cards can get scratched on the metal surfaces
can't handle anything that's not "standard height" - it'll do bridge and poker decks of the 3.5" tall variety, but it won't do larger cards, as they get stuck, nor smaller, as they don't stack up in the center bay.
crunches over-thick cards badly. In other words, 110# cardstock won't work well, as home cutting results in being too thick to work.
Kem and other solid plastic cards don't grip right - increased flipping, and more manual downpressure required
older machines' rubber wheels lose tackiness, and don't drag cards as effectively
not quiet.


All the automated ones I've seen have the same issues plus 


not stopping if a jam occurs
needing a source of electricity
being even noisier.


I've not used it enough to determine edge wear effects, but Kem used to note that shuffling machines voided their warrantees.