Everyone I know with a Shimano system has had it cut out with error codes at least occasionally. If you look around on here, some people experience this almost every ride. After they cutout, they can be re-started and will go again for some length of time, hours, or days without seeing the issue again. Unfortunately, they often cut out on a steep climb, which can really suck.
Several people on here have reported motor failures with Levos. Search and you will find them.
The Bosch and Yamaha systems seem to be more solid. I personally have put thousands of tough trail miles on my two Bosch powered eMTBs and have had *zero* issues. There are design flaws, IMHO, but as far as reliability they are rock solid. The biggest issue I've seen reported with Bosch that could be tagged as a reliability issue is that the Intuvia display, the large one that mounts in the center of the handlebars, is subject to damage in crashes, and if the mount is damaged, the removable controller display will become loose. When this happens, if you hit a bump, the display loses electrical contact with the mount and the motor shuts off. This is cured by either a new controller display or a new mount, or by just replacing the Intuvia with the later Purion model that lives on the left side of the handlebar, near the grip and which is not removable.
Same with Yamaha as far as I have heard, no one has issues with those, they just work.
The Shimano is Gen 1 and will probably get better, but no one knows when they will be updated. Bosch is just now coming out with the fourth iteration of their system, Yamaha is on Gen 3, Levo is on Gen 2.
