Bodhbh
Guru
At speeds over 25mph while on road, there does seem to be a certain twitchy feeling to the steering that I had never noticed before. However, that is the only thing I have noticed that could be deemed to be a drawback.
I came it the other way and stuck a 29er wheel on the front of a 26er (456). The frame can supposedly take 100mm to 150mm suss forks. I had some 100m suss corrected rigid forks on the front, but didn't really like the twitchy handling, and the frame was too stretched out, front too low for me - I didn't really get on with it. The obvious thing would've been to stick a longer travel fork up front, but it seemed overkill as I mostly just ride bridlepaths and the like. So I stuck a 29er wheel up front and it was spot on - brought the bars a bit closer and slowed the handling a bit. I liked it so much I later tried to put a 80mm travel 29er fork up there, but....too much of a good thing - it was floppy up front and started pulling wheelies on climbs. It had a non-trival effect on the BB height too - quite a poke to find the floor.