Bear with me on this, as it a long and rambling one.
I don't believe that any make of current crank drive electric motor (DU) as fitted to eMTB's is not going to be susceptible to outer bearing failure, especially if the bike is used in Europen off road conditions. You only have to look at how often bottom bearings fail on non electric bikes to format a reasoning that the same would be true of an electric motor that is in effect replacing that bottom bracket.
It is my opinion that until a manufacturer designs what could be called a pre unit motor, that some how separates the motor from that bottom bracket, that this will never change. The bottom bracket would still fail, but it will be a separate, mechanical component, rather than an electrical one.
Now to the point in question in respect of Bosch DU's.
Yes I have had drive unit failure, and below are my thoughts in respect of the system.
Firstly, if the motor makes it to 600 miles, then you are more than likely going to be okay. It seems to me that many motor failures happen within this initial mileage. The next stage is 600 miles to 2,300, and then 2,300 miles plus. I personally feel that sooner or later, that any/all of the current DU's which are used in an eMTB, will eventually fail.
This leads to the reason of why. Do you all recall the Bosch popping sound of the previous incarnations of Bosch motors. The reason for this was lack of grease within, and it was an issue easily solved. An issue that should never have even happened, and also one that Bosch denied liability of, for a significant period of time.
The current DU's don't suffer from that problem, but do suffer from outer bearing failure. Unless someone can tell me otherwise, I am only aware of the failure occurring on the sprocket side.
I'm not an engineer, but through years of working on motorcycle race bikes, I do have a thorough mechanical understanding, and it is my belief that the current use of the small size sprocket, is placing undue strain and load on the drive side, which is then creating fractional wear. This fractional wear, is then allowing undue moisture to enter the bearing area, and accelerating bearing failure rate. This doesn't explain the early mileage failure rates though. I have also noted that failure rates occur more on the bikes of fitter riders, where by lower power settings are used, and more strain is being transferred through the pedal cranks. I suspect that riders that aren't so fit, or riders that use higher power settings might not suffer so many DU failures. Again this doesn't explain the low mileage failure rates. Perhaps a conspiracy theory, but I also feel that eMTB mode was introduced to try to prevent these failures, by offering an assistance over and above the requirement of the rider, and so reducing strain. Despite claims, the system is not intuitive.
Another reason for DU failure, is poor cleaning teqnique method of the bike and it's DU. The fastest way to destroy the outer bearing (again the drive side) is the use of high pressure hosing down, and or the use of cleaning detergents and oils. I frequently cringe at the teqnique used by many to clean their DU's. I also cringe when I see dealers recommending using detergents, degreasers, and oils around any bearing area whatsoever. Don't do it!
The above leads nicely on to the laughable cure of the bearing failure issue. The Bosch outer bearing seal modification. I'm not going to hold back on this, as I think that it is a complete cop out, and a poor one at that.
The idea being that grease is first put behind this seal, and a plastic ring then pushed over to prevent water ingress. This will only work, if the owners removes this seal at least once a month, and re greases it. Even then, it is a touch and go situation.
Taking this one stage further, the seal is now fitted as standard, and I removed the seal on my current bike, after just a few weeks of use, and discovered that from the factory, no grease had even been applied (Back to my above statement about the Bosch popping sound and the lack of factory supplied grease) Are the sealed bearings even of suitable quality, do they have a suitable grease, and if the first prevention stage is being completely missed out, who knows! I now remove and re grease the seal very two or three weeks. Should I need to, and should I have needed to add the grease that the factory choose to leave out, no I shouldn't.
Also in respect of my current motor, the motor refused to turn on from new, and just kept shutting down. I claim to know very little of how an electric bike functions, but it doesn't take much to work out that there is either a software issue, battery issue, or power issue. Trying parts on my other bike, everything apart from the motor was quickly eliminated. Two things were left. Either the motor was duff, or there was break in the connection between the battery and the motor. I tried everything that could think of, but nothing. Then out of desperation, I shone a torch into the plug of the motor, and there was the issue. One of the pins had been bent flat during the assembly stage. I very carefully straightened the pin, and everything worked first time. The pin will now be permanently weakened, so who knows when that will fail, and the motor will die. Probably the first time that the plug is removed. I do wonder if that doesn't also explain the odd thread that you read of, where an owner receives a new bike, only to have it not turn on.
I'm fed up of people saying that I am biased towards Bosch. I do love the system, and I'm more than happy with both the performance, and the support that as a company that they give to all, and see no reason for me to switch away from the brand. To date, no other brand even appeals to me.
I'm not prepared to pretend that the design isn't flawed though, and until the company revert back to a conventional size sprocket, or carries out a proper re design of the current system, then I predict that these failures shall continue.
One other aspect to think about, is that no one has any idea of the quantity of DU's produced daily, so the actual the percentage failure rate could in relative terms, be close to zero. Obviously that is of no consolation to those that do suffer a failure though, and I'm certainly making no defence of it. To end, even with a motor re design, only a pre unit motor will prevent bearing failure.
My gripe isn't oddly with the motor, my gripe is with the batteries. The mileage that they give after just a year of use, declines quite significantly, or that has at least been my repeated experience. I also don't like the fact that as a company, they do not listen to customer concerns in respect of either design, or factory installed software. I have discussed this with Bosch de