And an A320 was very nearly lost after the Captains sidestick was miswired during recent avionics work - but does that mean we should do away with FBW? 'Course not.
We fly single-engine aircraft, sometimes over unforgiving terrain. Everything in aviation (and life) has risk associated with it, but just because there is a risk of something happening, does not mean it is probable, or even likely and so we should not undertake that activity (or install that auto-trim module). You seem to be assuming that there is no way to stop the trim during an inadvertent auto-trim movement, I don't know if that's because you're unfamiliar with the Garmin or Dynon systems, or autotrim in general, but pressing the AP Disconnect button will disengage autotrim, as will, IIRC, trimming opposite the direction the trim is running.
As for what's wrong with manual trim? There's nothing inherently wrong with it, but electric trim is much nicer to have under your thumb rather than twisting a knob while I'd prefer to have my hand holding the throttle full forward (in the case of the RV), it is often lighter and able to be interfaced with the autopilot. For smaller aircraft like my old KR2, the little manual lever on the longeron that connected to a bit of wire-in-tube to the trim tab was more than adequate. But as things get bigger, technology takes precedence.
Case in point...I'm installing EFI instead of the old RSA-5 system, but a risk assessment shows that I need to ensure continuity of electrons, so that requires a backup alternator. Should I stay with the old 1960's era injection system just because it doesn't require power? Heck no. EFI is well (flight) proven now, enables instant car-like starts - hot or cold, it allows LOP, I can run 98 Mogas, and I'll have the ability to trim fuel to each cylinder individually to balance the EGT's and the system has an option that provides a backup computer and fuel pump should the primary fail.
The benefits outweigh the negatives and any risk can be mitigated. Same for autopilot, autotrim, or any other modern gadget in our aircraft.