Flute is quite a bit louder than a Susato whistle. The only whistle I’ve ever tried that can actually match a flute in volume is the Cronnolly, and it is much louder than any Susato I’ve ever played.
I agree there are times a quieter whistle is better. That doesn’t make Susato a bad whistle, just not a quiet whistle–same is true for the Cronnolly, in spades.
I like having many whistles, with many kinds of sound available. If they all sounded and played alike, whistle playing would lose most of its appeal.
It’s just that given the properties of a whistle, it takes much less volume to reach you. It’s high pitched, and for example sitting beside a Susato player could actually hurt your ear, but not beside a flute player, because it’s an octave lower. A whistle, even quiet, can slash through a session, without much volume, but you can lose a flute quite easily if there’s other flutes and fiddles around. Also, two whistles and more in a session can actually wreak havoc, but more than two flutes will sound much less “in your face” and aggressive. I think this feeling is shared by a majority or musicians, at least those with older styles.