What do you guys think about that ?
it all depends on your personal refereces in terms of sound.
for example, if you like an 80's frizzleing bright kind of sound, like Mark King or similar things, a "Club Tropicana" sound, stay away from flats! they won't work well.
if you don't have extreme preferences, you can play whatever you like. sure is that flats won't give you the harsh highs of new roundwounds, but with modern style flats (Chromes? TI?) is easier to get away with a bit of EQ.
in my personal view, flats are actually more versatile by themself, fitting in every context without any issue, and always with the right amount of bass and mid frequencies to cut through and sound like a bass.
obviously, if you need to play for a metal band who wants a super-scooped sound with extreme highs, flats won't fit... but usually in those bands the bass player uses lots of pedals, like preamps with distortion and EQ, through distorted amplifiers, and even flats can sound crispy and "metal".
I played with a modern-grunge band using a Yamaha BB415 strung with Thomastik JF345, through a Fulltone Fulldrive 2 MOSFET and a Marsall JCM2000 (on a small Ampeg 210av

wonderful cab, better than the 4x10 Marshall), and the sound was KILLER!
the fact is that we usually associate flats with music played in the 60's and 70's, and with high action, foam under the bridge, tone rolled off... I set my flats with the lowest action possible, because there are no "ziiings" (because their vibration is tighter) and they sound wounderfully slammed on the frets, like a bass digged in, and not like a guitar.
once I went to the reharsal with rounds, and everyone of the band told me that my sound was sh*t, and that I sounded like a guitar... and I used to play modern music, not vintage style things...