Yes, people have been refused FLR. Even some members here have had to endure the stress and anguish involved in facing that challenge.
Just let's be realistic. Under normal circumstances FLR approval is a formality.
Almost all the FLR refusals I have known were eventually resolved without anyone needing to leave UK and all were highly unique circumstances.
If your fiancee visa is granted all you need to do is to maintain the same (or better) compliances in order to secure FLR.
Means don't allow anything to fall below the levels (eg financials/accomodation) that were submitted in support of the fiancee visa. Remember your fiancee will not be allowed to work until after FLR is approved.
I hope that puts your mind at ease.