It's kinda like a regular MTB race - you may have one guy on a 43 pound department store rigid bike and one guy on a 21 pound carbon full suspension wonder bike...
Much like some forms of racing, there are specifications you need to conform to:
- Class 1 eMTB (assist up to 20 mph only, assist only when pedaling)
- Max peak wattage 750w (most are less than this)
- No modifications, alterations, or "cheats" to the speed regulation system
Even with that, there are enough differences in the current eMTBs that do make for some inequity, particularly on open, straight sections of the course. That can somewhat be compensated for with a more technical/tight course.
The only real way to make it perfectly "even" would be a spec series, similar to car racing, sailboat racing, etc...but that's too limiting.