In 2006, I redeemed for two nights in London. No problem.
Earlier this year, I reserved 6 nights in Ireland. Again, no problem.
I think you might run into a sold out property every now and then, but I haven't run into it yet.
BTW, there's a list (compiled by me) of all the Preferred hotels in Europe. I have updated it to include all Preferred and Summit hotels world-wide (might not be posted yet)
Ooops, forgot the link.
Spreadsheets
here.