It's best to have all the facts, then you can make the correct decision.
I agree with both of you!
If you have a two year guarantee on the motor it is very reassuring if you can see that the company in question will replace the motor with no quibbles if you have a problem within that time.
My Yamaha motor in my crank drive sDuro developed play in what on a normal bike would be the bottom bracket and I could tell that there was a good couple of millimeters. By laterally pushing and pulling the crank arm opposite the chain ring I could see that the chain ring moved more than it should do.
The shop I bought it from (ebike shop Farnham) replaced the motor just after two years of ownership with no quibbles very quickly with no charge. The motor had covered 6000 miles when the play became noticeable.
I also own an older 2011 Oxygen Emate City that Andrew at Oxygen replaced the motor wheel for me just inside the two year guarantee and that second motor is still working well.
However it is also very useful to have empirical data about any given motor and how reliable they are statistically. How many are likely to fail and how far can you expect them to go on average before they fail or require some serious maintenance or replacement.
I am sure that the bearing that developed play in my Yamaha motor could have been replaced but it was obviously easier to swap out the motor and I was very happy to get a new motor very quickly with no quibbles just after two years of ownership.
All crank drive motors must have some vulnerability in the main bearing that handles the force of the riders effort through the pedal arms. This vulnerability will be effected by the quality (needle roller bearings etc), size and design of that bearing and the power that it has to handle. Weight of rider!
Personally I would prefer the strongest highest quality bearing possible to increase the lifespan of the motor even if that meant a slight weight penalty. However I use my bike to commute and do a relatively high annual milage and someone else might be happy to trade some reliability for less weight and more power.
Impulse owners might just want the bloody thing not to break down! Though we do not have any real statistics as to their actual reliability.
