I've used flats extensively on 5-string basses and, in the main, they are absolutely fine.
My favourite flats are LaBella FLs and they work with 34" basses though the B-string can be on the short side.
I now play exclusively 35" scale, and the FLs are simply not long enough. I use what I think of as Lakland Jo Osborne flats, but Lakland now call them something else and Gareth (Walbassist) assures me that they are anyway just re-badged GHS strings.
With 34" scale on a 5-string there is definitely the risk of some flobbadob going on, so you also need to take into account your own playing style. I like a high-ish action and I tend to dig in pretty hard.