I see it like this...from the perspective of not being mechanically inclined.
I did my home work and bought a compact Sony camera approx 3 years ago it was the best at that time I think and about £250? I still have it and it works perfectly. Over the same period my daughter has had three and now needs a fourth. she spent £50 - £80 on each. I think I will stick with Wisper for the same reason.......
Ahh yes, but your daughter probably doesn't take the same care of things as you do because she buys cheaper.
Looking at it from a different angle your daughter has had the variety in her purchases of 3 or 4 different camera's, hasn't spent much more than you and £80 today buys the technology that £250 did 2 or 3 years ago.
If your talking about digital camera's her newer cheaper model will probably have higher specs than yours, its exactly the same with mobile phone technology etc continually improving.
Some years ago I bought a £800 digital camera and all the extras to go with it. I took out an extended warranty. After a couple of years I accidently dropped the camera on the kitchen tiles and broke it. When I claimed on the insurance they replaced it with one costing £147, the reason being the policy stated a replacement should be of similar or higher specs and the replacement exceeded the specification of the original. I managed to talk them into going a little higher on price because none of my accessories were compatible with the £147 one.
Ebike technology is pretty basic, the bike itself is ancient technology and little goes wrong, the motor controller and battery are the main concerns and are still improving, newer 8sp hub gears are great but not user friendly because if they fail you'll probably be walking home. If shimano's fail you can generally fix them in one gear and continue on your way.
Regarding batteries, even the best can fail and sometimes the cheaper one's excel. Brushless motors have little to wear apart from bearings and the windings can burn out but their pretty reliable. That leaves the controller which I think is the weak point of ebikes, a mass of ic's, transistors, diodes, capacitors and a circuit board. This can be affected by heat, moisture and shock which should all be taken into account during manufacture. Failure of the controller will almost certainly mean a replacement because its not cost effective to repair.
Biggest causes of breakdowns are punctures and chains breaking I would think which are common to all bikes irrespective of price.
Reliability is important but it can't be guaranteed, even the best technology fails. Having a spare would be more reliable than just the one which is why buying cheap works.