[quesera]

> Troubleshooting an intermittent problem on a bathroom fan in a subdivided triple decker with common grounds -> hard

I would generalize that to "locating and safely fixing problems inside closed walls or ceilings -> sometimes hard".

Sometimes easy still, but that is where things get complicated, and sometimes dangerous. I've found dead-ended intermittent lines left inside walls without any attempt at proper termination, for example. This is how fires start.

Doing your own electrical work exposes you to the lazy errors made by other people in the past. You learn not to be those people (some of whom were licensed electricians).