I think the main issue is the backup, not so much the motors themselves. In my opinion the motors we have at the moment are pretty much where you want to be in terms of output, and the main advances are in the software, which is after all what determines how the bike feels to pedal, and how it reacts to the riders inputs. The software is also what makes sure the motor doesn't eat itself.
As
@steve_sordy says the failure rate of some of the motors, in terms of timelines, is on a par with a lot of normal bike components, the main difference is that in the majority of cases, mainly due to lack of experience in some LBS's, and also what manufacturers are recommending, most problems, even small, are solved by a motor or battery swap, and currently this takes time as the motors have to be sent back to the supplier, certified for warranty, and a new one sent out.
Now there are some dealers for Specilized, such as Berkshire Cycles, who hold stock of spare motors, and make this process a whole lot less painful, and on the Bosch side of things they hade service centres at certain dealers where they can repair various problems on site.
I love my Vitus, and the issue with the motor is frustrating due to the timescales involved, just to fix the torque sensor.
The one thing I would say from nearly 2 years of EMTB ownership, riding hard in all conditions, all year round, pretty much every other day, is you need to stay on top of maintenance, and that I will defiteyl buy my next EMTB from a dealer, and a dealer that has the necessary experience so that they can put the bike together properly in the first place, and the back up to sort issues quickly.