A close-range salvo discharge from a massed infantry unit was a nasty thing. Very nasty. The musketeers would of course be kind of screwed if the foe didn't falter and break off and they lacked close-in defenses (like pikemen or cavalry of their own - later on bayonets simplified matters), but that wasn't actually too common of an occurrence.

Or, well, depends on definitions. When infantry attacked infantry there tended to happen a queer version of the game "chicken" - as the discharge was only really effective from fairly close in each commander had to hold his fire as long as possible, to maximise the effect and avoid the nasty case of the foe marching right next to you to fire his own salvo while your guys are helplessly reloading, but if they held it for too long...

Well, at close distances those volleys tended to cut people down like so much grass. It took a pretty determined unit to continue advance in the face of the heavy casualties in the front ranks and the psychological impact, and most had to pull back to reform.

The same more or less applies to cavalry, who had the added problem of the horses being big and squishy targets, and duly tried to avoid head-on clashes with well-ordered infantry. If they had to attack, it usually happened (assuming the infantry officers knew their stuff and the men followed orders) they'd get a point-blank volley in the face which usually made the first wave of horsemen break off the charge. The second and later waves had a better change of charging home, but that was not something to count on either.

Around Napoleonic times cavalry avoided head-on collisions with steady infantry if at all possible, and let artillery and skirmishers to "soften up" the line before attacking. The same more or less worked with infantry - the deep assault colums the French used early on, mostly because they didn't have the time to drill their troops in the volley-countermach routines, were frightfully vulnerable to volleys but almost unstoppable if they could get into close combat (due to local numerical superiority and the advantage of momentum and determination attacker has - though usually the defender broke and fled before the actual contact). Hence the voltigeurs, loose-order light infantry who screened the line troops, sniped at the enemy and if possible drew their fire (whose effect was much reduced against such dispersed targets).

As such loosely ordered units could not rely on the "giant shotgun" principle of the mass salvo to have an effect they naturally had to be better shots on the individual level.

And then there were the light regimental guns. The nasty little buggers had a far longer accurate killing range than musketry, could especially in a pinch fire several times faster than muskets, and normally switched over to grapeshot once the enemy was within about hundred meters. I assume imagination can supply the idea of what those could do to massed formations.

Proper artillery batteries tended to need only a handful of infantry as close guard - their firepower was so staggering they could usually fend for themselves right well for entire battles.