There can be problems with all types of motors, I have done 1000s of miles on bikes with Bosch motors and had no problems, The problems I have had have been with hub motors, mainly the Carrera Crossfire.
Of course there can be problems with any motor type, but don't forget, an average bike DIYer can repair or replace a problematic hub motor, even replacing the whole wheel and motor is fairly easy. But for example, Bosch will not even sell a private person motor spare parts, or even a motor. You are completely stymied!
The Bosch motor has to be replaced by a licensed Bosch LBS, and as has been reliably reported here, an exchange motor costs around 800 UK Pounds from Bosch!
A few quid more and you can buy a brand new hub motor bike, complete! Mine was around 930 Pounds, including two batteries, import duties and delivery costs.....Nothing fancy, but very reliable!!
Then with the Bosch motors/bikes, you have the problems with the CAN- Bus, which for every slight hiccough, can shut down all battery power and make even buying good, secondhand spare parts, impossible to use, because the Bosch controller via the CAN-Bus, will not recognise a different Bosch battery for instance....
One has to compare horses with courses......and mention all the plus and minus points. I do and I try to be both completely truthfull and fair.
And we should never forget that the stressed chains on mid motor bikes have a far shorter life, even when carefully maintained, in comparison to a hub bike's chain.....in spite of the many protests here on Pedelec from mid motor fans, but it is a simple fact of life, no matter what they say.
Simple physics.
I also dislike the fact that Bosch use plastic covers of the very cheapest quality, and all Bosch motor parts are made in the far east, just like my cheapo bike, maybe even in the same factory, but one look at the prices new of Bosch motorised bikes, does take one's breath away....
Rant over.
Andy