My motorcycle ortliebs have been fine, the straps over the seat/top are plenty long enough on my two suzukis, might not be on a wider BM though! I wish the straps were different where it connects to rear pegs or such like, as there is too much movement for my liking, and the strap excess is to long, I have to cable tie these out the way, which makes removing the bags quickly a PITA. Had no problems what so ever with waterproofness. They do the job well. Not really suitable for hoiking off the bike every night, easier to leave them on.
If they go anywhere near or above your exhaust, invest or bodge a heat shield though. It doesn't take a lot to make a hole in these, as an officer of the law pointed out to me with frantic hand signals whilst passing on a motorway.
What proportions of tarmac/rough stuff do you intend using them for?
'Life begins at the end of your comfort zone'-
UK to Asia on a DRZ400 2012-2013