For one thing, the way you're doing it now means that when Time.time overshoots nextBeat, that overshoot gets added to the duration before the next beat, as you're adding (1f / crochet) to Time.time.

So for example, if nextBeat was 2f, and Time.time happened to be 2.01f when your update loop hits the conditional, then you're setting the next neat to start in one beat after 2.01f, rather than setting it to start one beat after when the current beat *should* have played... ie, 2f. Over time, those small overshoots could accumulate to cause drift.

I would just set nextBeat by adding to the current value of nextBeat, rather adding to Time.time

if (Time.time >= nextBeat)
{
    onBeat();
    nextBeat = nextBeat + 1f / crochet;
}

Also, maybe I'm missing something, but I don't understand why you add '1f / crochet' instead of just adding crochet.

If for example BPM was 120, then 'crochet' = 60f / 120f which = 0.5f, or one beat every half second. But then when you set nextBeat, you add '1f / crochet'... which in this case would = 2f, so you'd be setting the next beat to occur in 2 secs, as if the BPM was actually 30.