More complexity to the mid drives and if your motor chain breaks, or something (power, trail dmg) breaks your drive train have fun walking.
Changing gears on full power can break things - there is the sensor you can install on some of the m.d's. I've noticed on some hill climbs on my dd hub I need to change gears. When I am getting to the hill at w.o.t. in rear 34T and get half way up on w.o.t. and need to change to a 20T half way up then lowest T of 14 or 11T rear gear still at w.o.t. to get to the top - any pause in the power applied while going up the hill to reduce power on mid drive to change gears will have great affect on getting up the hill. It comes down to make or break getting up the hill. But yeah sure I've done C4K from a standstill on a 20% hill, and able to climb in the right gear.
Besides all that, "mountain bike riding" is a very broad term.
- Goat trails with rutted roots, cut tree trunks
- Dirt bike trails
- Tight trails
- Easy swooping corners of a ski hill bike trail, like velvety trails of green/blue runs
- Big jumps like drops off made platforms of ski hill bike trails.
- Small jumps
- Back country forestry roads, typical gravel that any mtb can do
- Single track with various conditions from technical turns, to easy lumbering look at the birds trails.
- Large chunky rocks, like dry river beds or puddled or riding across a river/stream.
Heck "mtb riding" could even mean riding high altitude in the resort towns easy bike paths/sidewalks/roads, hey your in "the mountains"