Oh, no question that's true. The point is that getting that classic, "electric Stanley" sound - not just the piccolo bass sound - but to get that ubiquitous, Alembic, clean, bright, clear, in-your-face, electriClarke sound - flats are unfortunately unable to get in on that territory.
But as for being very versatile and surprisingly broad - flats are definitely able to provide that. I think the "problem" with flats for many is that because a lot of basses sound 'better' (brighter, more instant gratification) with rounds, when people play a set of flats they tend to feel like something is missing - and something is - but it's not a bad thing that it's missing - but it 'feels' like it is...
You need to prepare your mind to adjust to the nature of flats and wean yourself off of the instant gratification of rounds before you can fully appreciate all that flats can do.
Does that make sense?