We work on ebikes every freaking day and in general it is usually the 'user' that is the source of the trouble. All too often the user rides it all day every day and does absolutely no maintenance. They also ride them far too 'hard' for their intended purpose. This results in badly worn brakes, etc. Also many riders just grab the bike anywhere when trying to drag it up stairs, etc. and wind up damaging wiring because they grabbed it by the bars and wires and tried to drag a 70lb bike up a set or more of stairs.

Trying to get 'non bike' parts is very difficult as the seller just 'sells' bikes, not replacement parts, requiring the owner to go the maker direct which always takes time and numerous emails.

Very often the maker changes the design of the electronics meaning the 'new' part won't work with the old part...now it's a lawn ornament and winds up abandoned.

If the user takes care of it, doesn't use it like they just stole it, etc. it will last a long time buuuuuuuuuuuuuuut that is seldom the case regarding the ebikes I get in every day.

Oh and make sure you can get the correct tubes and tires...many require tubes with an 'angled' valve stem and they are not generally available in the bike shop and the owner has to order them from online suppliers...the owner, at least in my experience, seldom orders the correct size and/or stem configuration causing delays and aggravation on everyone's part.
Same with tires...very often they are in odd sizes bike shops don't carry and don't have access to from their normal vendors so again the owner has to get them and the same problems occur as above.

good luck...