Hmmm... not had one done personally, but I've seen a couple. I'm going to go with.... very limiting. Your ankle is bolted together and becomes completely solid, so you lose the flexibility you need for cycling. That said, you'll still be physically capable of cycling, down to the shops and whatnot but yer more advanced stuff will be tricky..... If it helps visualise it, then walking is a bit funny afterwards too, they give you special spongy inserts for your heels to try and make it as natural as possible, but you've still got to rock over your foot if you get me.
Arthrodesis should always be a last resort, specifically for these reasons, is a TAR - total ankle replacement not an option?