Q:

Canyon Spectral:ON different wheel size (29''/27.5'')

Canyon Spectral:0N boasts a different wheel size: it has a 29''/2.5'' as front wheel and the 27.5''/2.8'' back. This difference in wheel size should make the bike more nimble and the extra-wide tyre back should maximize traction on difficult slopes (traction for going uphill and breaking for the downhill I guess). The smaller diameter wheel on the back leaves the space for a shorter chainstay, thus more maneuverability. All the reviews I found on the internetz are quite positive.
I was wondering:

Anybody has experience with different wheel sizes?
Any considerations for pedaling on level ground with this setup?

A:

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.

A:

Anybody has experience with different wheel sizes?

You can look at the World Cup competition-level athletes running the "mullet" setup. Just use net search for "mullet bike" and "world cup". With a caution in regard to limited statistics we have now (UCI has allowed different wheel diameters to be used on sanctioned events relatively recently), what I can say is that the 29/27.5 setup does not hinder top riders to remain at the top. E.g. this. Whether or not the mullet actively helps them to win is too early to say.
The story is the same as with any previous new technological advancements in the sport, such as front suspension (stayed), rear suspension geometry (URT died but e.g. Horst link stayed), plus tires (dead), dropper posts (stayed in certain disciplines), 29 inch wheels (preferred by taller riders), frame geometry changes (a lot of experimentation seen over the years) etc. etc. Basically, time will tell.
Even if the mullet setup does help a few best athletes to win, how their performance translates to what regular people would experience on the same bicycles is even less clear now. Other factors, such as price, maintenance cost over time, and trendiness are as important to regular consumers as the actual performance on the trail.

Any considerations for pedaling on level ground with this setup?

Look at the geometry charts and compare the numbers against a regular 29'er or 27.5'er of the same size from the same vendor. Look at the values of e.g. BB drop, BB height, standover height. The mullet setup does not mean that the manufacturer just took the previous year's frame and slapped different wheels on it. There must be some tweaks done to make the bike rideable in its intended discipline.
But, if anything, this bicycle is not meant to be ridden on level ground much (where is fun in that?). Especially that the mullet setup is not supposed to make any difference in that case.

