Switch/breakers have a tendency to fail quite a bit in my experience. The thought is that it is related to vibration, but that may just be the particular type of breaker/aircraft.
I wouldn’t know why you would use non-pullable ones.
Question: is it wise to pull a circuit-breaker to simulate a failure during a training flight?
Having spent all day replacing the buggers with new Klixon ones, I tend to agree 
I think the reasons the CBs fail is because they get hot. That is how they work, after all. A 10A CB carrying say 8A will be pretty hot inside.
A really back-breaking job, because one cannot get to it. I found the least bad way is to sit on the wing and bend down and to the right. One could lie down into the LH well (obviously the seat has to come out) but then you don’t have room for a soldering iron and a hot air gun.
About half the wires had to be extended.
Tidying-up to do…
The original Socata (ETA) CBs will be for sale – for €10,000 each and with no paperwork 
But, hey, there must be a way to recertify a CB… I was on the phone to one 145 aviation supply company 3 days ago and nearly all their Klixon stock was at least 10 years old, some 20, and they said “it is surplus stock, they were likely 10-15 years old when we got them”. But evidently their customers are OK with this. More likely they never realise.
Left over:
