I believe that he refuses to fix Shimano motors because the circuit board is neither a serviceable nor replaceable item. It can fail at any time and he got fed up of customers complaining when the board failed a few weeks after he serviced the motor. It was costing him too much in money and good will.
I think it was a bit more complicated. If he made a repair to one problem in the motor (when Shimano refused to cover it) then Shimano then held them liable for any other failures in the motor and wouldn't cover them either. So with a motor with several known issues it meant Shimano just found more ways to wash their hands of the problems and make their warranty figures look better than they are - at the expense of owners and anyone who was actually willing to make repairs.
It's not that it's an awful motor. Or that it will fail on you. It's that it has several issues that have a higher chance of failing than they should and more importantly, Shimano's motor division's policy to try and avoid warrantying motor's or batteries wherever possible. Which as we see, is long term detrimental to their own business model as more and more companies jump ship from Shimano to other suppliers.
Years ago, when there was mainly just the early Brose's and the Bosch Gen2, Shimano's E8000 was an exciting alternative and lots of bike companies understandably developed models using that option. Now, why would you ?
Yes, the Brose has been renown for failures. But a large part of that was just because of the vast numbers out there and a vast amount of failures are caused by pedal strikes which don't immediately fail at impact, but cause the belt and/or sprag to begin to fail.
The flip side of failures and large numbers is that it became viable for third party companies to setup to repair them and manufacturer or obtain the necessary parts to do so. Hence, due to them being sh1t

, Brose motored bikes are now probably one the safest second hand options to buy and at least buying new, you know you can get the thing repaired down the line if you decide to keep it longer and know that you can sell it, because people can get the motor repaired.
The Gen4 Bosch is generally pretty reliable and again you can get it repaired. It's not without it's faults longer term especially in wet environments, but you can change/upgrade seals and grease to help with longevity.