Well most of your weight shifts to the front of the car during braking which is the reason that you find much larger brakes on the front wheels of a car vs the rear. As such, there is only so much braking you can get out of the rear wheels of a vehicle before they overcome traction. This becomes even more true if it's raining as there is less traction afforded the rear tires as weight shifts to the front of the vehicle. In a nutshell, you can be much more aggressive regen braking if you have a motor on the front wheels.
This is all true, but not actually relevant as a limit for EV regen power. The load transfer is a function of the deceleration rate and CG height. A car's brakes are generally sized for rapid stops (~1g), which create the most transfer, hence the larger front brakes on most cars. The other extreme is a slow stop, which has proportionally less load transfer, so during a typical stop (0.1-0.2g) the front and rear tires see very minor transfer and their braking effectiveness is dictated by the static weight distribution to each axle. The max regen seen in EVs is around 0.3g, with the Model 3 having an average of 0.19g in the standard setting.
At such relatively low decelerations and with the low CG from the battery, the load transfer is not a significant factor that would limit regen power. Both FWD and RWD EVs have a very similar ability to create regen power in a stop.
If we assume the Model 3 CG height is 18", the same as the S, then along with wheelbase we can calculate how much load transfer happens as different deceleration values:
Example:
CG height: 18" (est)
wheelbase: 113"
3 LR weight distribution: 48% Front, 52% rear
CG longitudinal position: 59" [113*.52]
Rear wheel load at 1g: (59-18)/113 =
36%
Front wheel load at 1g: 1-.36 = 64%
Rear wheel load at 0.3g: (59-0.3*18)/113 =
48%
Front wheel load at 0.3g: 1-.48 = 52%
So during a hypothetical stop at 0.3g, only 4% of the load shifts forward. Since the Model 3 starts with a 2% rear bias, the load shift to only 2% forward bias.
One more example:
3 SR weight distribution: 47% Front, 53% rear
CG longitudinal position: 60" [113*.53]
Rear wheel load at 0.19g: (60-0.19*18)/113 =
50%
Front wheel load at 0.19g: 50%
For during a standard regen stop in the 3 SR, rear tire load reduces from 53% to 50%.
Ok, one more for giggles:
Jeep Wrangler weight distribution: 50%/50%
CG height: 26" (est)
wheelbase: 95"
Rear wheel load at 1g: (47.5-26)/95 =
23%
Front wheel load at 1g: 77%