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.