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.
