Canyon Spectral:ON different wheel size (29''/27.5'')
No matter what the reviews say.
No matter what anyone tells you.
You need to ride this bike and decide for yourself if this bike does what you need it to do.

According to Canyon's Spectral:ON spec sheet they are running:

27.5 X 2.5,  63-584
29 X 2.6,  66-622  

From bikccalc.com
Rear:
Rim:650b/27.5 Tire:2.50 inch Rim (iso):584 Tire:63.50 Wheel diameter:711.00

Front:
Rim:700c/29er Tire:2.60 inch Rim(iso):622 Tire:66.04 Wheel:754.08 (rim diameter plus 2 x tire)

754-711=43mm difference in diameter.
The chain stay runs the radius of the wheel so it is theoretically possible for the chain stay to be 21.5mm shorter than it would have been with the same size wheel.
Can you feel 21.5mm on a full suspension electric bike?
Only you can say for sure.

Smaller rear wheel has been done before, more than once, on a mountain bike.
Here's a picture of a 1988 Cannondale SM 600 with a 24" rear wheel and a 26" front wheel.

(I know it's not the same thing as a full suspension electric mountain bike)

I use to ride with a guy who had a bike like this. He didn't love it enough to keep it longer than a year.

The 2009 Trek 69er 3x9 had a 26" rear wheel with a 29" front.

Here is an interesting discussion on bikes with different wheel sizes on the Bicycle Stack Exchange.