Because flatwounds carry less upper-frequency content, their decay can sound and feel different, but a well-made flatwound set can still give you plenty of sustain.
Flatwounds are often perceived as firmer or stiffer, but two strings with the same gauge can feel different depending on their core, wrap, materials and construction.
Flatwounds aren't dead strings, either. They're designed to produce a smoother, warmer, more controlled tone than roundwounds. Players new to flatwounds sometimes mistake that darker voice for worn-out strings, but it's part of the design.