While many frames don't have ideal bottom bracket areas suitable for a mid-drive motor conversion you can improve your chances by choosing a very overbuilt frame and if the worse thing happens and the frame fatigues and fails a secondhand replacement frame for very little money is available. People have destroyed their proprietary e-mountain bike frames through an accident and just the frame could be anywhere up to £2000. I've seen a Calibre Bossnut converted to a mid-drive e-mountain bike and it was performing brilliantly and the whole build was less than £1500 as the Bossnut was bought secondhand.
I personally wouldn't touch a proprietary e-mountain bike with a barge pole unless it was practically given to me.
I guess I would consider a Bafang mid-drive ebike if it was using a fairly standard mounting for that company, i.e. future motors would use the same mounting and they supplied spares to end users and it could use any battery but from what I've read on this forum Bafang are going more toward proprietary probably because perhaps they are pushed towards it by the companies that import Bafang based e-mountain bikes and want to control and profit from after-sales support.
I was in a bike shop looking around and I overheard a customer who told the salesman he doesn't want a bike with anything proprietary as his last bike he couldn't get parts for, it was actually a road bike he was looking at. As far as I can tell he ended up looking at the entry level models only, because he wanted a standard seat post, stem etc but I think he stated a budget around £3000. He left the shop about the same time as me without making a purchase. This was maybe 18 months ago. Proprietary can really lead to a bad experience. Hopefully that cyclist found something in his price range elsewhere perhaps from a smaller brand without all the proprietary parts. There was also a youtube channel of a cyclist who bought a Specialized road bike with a future shock he couldn't get spares for and his youtube channel was taken down after he complained about the poor experience he had. I think 'value' and 'proprietary' are mutually exclusive.