I have built 7 ebikes - both rear hubmotor suspended mountain ebikes and front hub motor non-suspended.

I have found the rear conversions to be less than satisfactory...

a.) More complexity with drive train/spacing/disk brake issues.
b.) Rear rack must be fortified or braced down to seat tube, as it is not connected to near the dropout.
c.) Forces batteries to be mounted in frame as too much weight on back on questionable rack support otherwise - all of the weight would be on the back - motor and batteries. Very unbalanced.
d.) Can't use internal hubs such as Nexus or Nuvinci.
e.) Not really good for normal panniers with side-to-side shifting of loads possible even with fortified seat racks.
f.) Too much bobbing around with all that weight on the suspension.
g.) Easier to have a removable battery solution for use on multiple bikes in a frame which has no battery "well" ie. a non-dedicated ebike frame.

The front hub motors are easy to install and are nicely balanced against a battery on a rear unsuspended rack. The battery can be put in a wire office tray on the rack to give it some side impact protection. Or the battery can be put in a pannier.

I also have a recumbent and a 20" folder with front hubs, they are both much better and easier to build than the mountain bike with the rear motor, and the battery placement makes much more sense (under the seat low down on the recumbent, or on the rear rack of the folder). Good balance.

If you are pedalling oriented rather than just building an e-motorcycle, the batteries in the frame or in the diamond are a painful struggle to realize and rather get in the way of pedalling.

Much prefer the front hubs - need two very good torque arms, no alloy forks, decent dropouts...

I use 48V 15Ahr Ping batteries in a Plano 1412 waterproof marine box (fits perfectly with 1/2 inch closed cell foam inside the box around the battery except not to smother the BMS), placed and bungied into a wire office tray zip-tied to the top of my rear rack on the folder - easy to remove the battery and very well protected if the bike falls over. I open the box for charging, due to BMS heating during charging.