I put a 1000w bafang mid drive on my mtn bike. Because the additional weight can't be engineered into specific locations on the bike, the balance is off. It will still get air, but it doesn't feel as smooth as it used to before the motor. I also don't think the kits can hold up to the rattling wear and tear that true DH mountain bike rides have in store for us.
With that said, I ride it almost everywhere I used to ride before the motor. I dont really do a lot of jumping. I'll get a little air where I can, but nothing I'd share on YouTube unless it was for a laugh.
If I were planning on doing downhill trails and jumps frequently, I'd go for a OEM bike