Make them set the fuse manually as well, they were marked to the nearest second IIRC. Also some chance of them not going off at all. I wouldn't be keen on it for all ships, just those that historically used is as a matter of course. There are other ordnance options missing too, double Ball with grape on top (when double ball just isn't enough...) bar shot, heavy grape....
I would keep fused rounds limited to mortars, nice differentiator, not sure there is enough historical widespread use as a precedent for anything else.
again just my tuppence worth,
Cheers,
Alan