We all know flats (esp on a P bass) are the cat's meow for staccto playing (e.g. mowtown) but what about 1/2 and hole notes? Do they work as well for that or does the sustain and midrange clarity of the round wound string save the day on those long notes?
They'll be fine for legato, and I'd argue they work really, really well. Especially if you're like me and come from a classical music background so you know how to really "milk" the nuance out of the strings.
Well I'm not from a classical background (as evidenced by my misspelling of legato (fixed)). Most of the online demos I see are busy parts or are done in isolation so you don't get to hear the way things sound in the mix.
Legato playing really has a lot more to do with technique over the strings you use. Use a bit of vibrato, tie some notes and watch your attack when you're changing will all contribute to a good legato feel.
Plenty of flatwounds that work nicely for longer lasting notes too, despite the myth about flats being all about thump. You could also argue that the existence (and use) of different dampening techniques (felt pieces, etc) is indication that even flats can sing. In my experience, some of the flats with decent sustain are Thomastik-Infeld, LaBella 760FS, Sadowsky black label flats. I have played blues gigs and even rock cover gigs with those flats on a P, and it all sounded nice, even the less busy numbers. There are probably lots more too, but those are the ones that stand out to me, I've tried 90% of the stock flats you can buy online.