Short answer - both can contribute.
Long answer, read on.....
Well, now that you ask, this is one of the reasons why I changed my motors to the bought T-motors, I have never had one of those fail in any of my craft, so I like them - but - I do take the time to maintain them as well.
I really didnt like the noise coming from the stock motor bearings. I have had experience with ESC and bearing failures in the air (fixed wing electric UAVs) years ago, thanks to HKing and i will never touch that brand again.
The pod ESCs are fairly standard with SimonK fw with 6 x nFETS that are rated to 18 amps (or 20, cant remember) no heat sink, but the top fets do have air flow across them when the motor is spinning. They are very low resistance and dont really produce any major temperature rise in normal operation.
Given that in flight each pod is only drawing around 4-5 amps they are rated high enough, the FETs are a good enough choice.
Problems start if the Solo flips or the props hit something whilst the motors are still driving. The current will spike and can partially (or fully) burnout a FET in a pod due to overcurrent.
Then you can have problems, if a FET is slightly burnt it may work, or it may not, or it may be a time bomb waiting to fail.
I have had one pod esc fail on my thrust tester when i was doing the prop testing a few months ago. It didnt just stop, it is intermittently losing power and at times 'pulsing', which is indicative of a single FET failing.
In the air this would cause bursts of uncontrollable yaw and diagonal tipping toward the faulty pod. I have not flown this pod, it was bought off eBay for experimentation. I intend to fix it and report the results here for anyone who is interested (its one of the jobs in the queue)
I read about these motor pod failures and I wonder how many of them had previously had a prop strike event, or a bearing failure or seizure, because I think these are the two main reasons that will lead to a pod failure.
I dont want to overstate the problem, and Im not criticizing the manufacturer - they are doing an amazing job at customer service and taking care of customers with these issues.
When a pod fails in flight on a quad, two things happen, yaw control is lost and the craft starts spinning out of control, and the quad will usually diagonally flip as well with the failed pod going down and the opposite corner rising up, usually a spin and flip in combination.
A CW spinning prop will cause yaw to the left and a CCW prop will cause yaw to the right, usually this is all in balance and the flight controller adjusts prop speeds to yaw gently left or right..
If, for example, a CW pod fails, the two CCW props cause an abundance of right yaw and the quad will initially spin to the right, (and then probably start flipping as well).
There are too many videos out there of Solos spinning and crashing, and if you slow down the footage you can usually tell which pod is failing, by the direction of the spin and the corner that tips down on the initial flip.
For me, replacing the motors is a decision I made to try and 'avert problems' as
@Maddog puts it (and so far so good) but if my T-motors now made as much noise as my stock motors made originally Id be changing the bearings. I don't know how much warning small bearings give before they fail, because when they are noisy and vibey, they are telling me something is not right inside an they need to go. And as discussed in a another thread, changing the bearings in the stock motors if they become noisey is good enough. The motors are essentially ok, just the bearings are poor. I dont see any real problems with the ESCs, if they aren't given a hard time they are fine.
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