> If you keep the same recoil, kinetic energy scales lineary with muzzle velocity.

This is true, and high-velocity projectiles would also penetrate armour better. However, they also penetrate flesh better, to the point where they just go through and don't do much damage. Still unpleasant, though.

> The usual 90%-reflective-household mirror stuff is no obstacle for a proper laser beam.

On a battlefield, it would matter, though. However, mirrors get dirty, and degrade as they are damaged. Just blast away until you've burned away the mirror.

Internally, lasers use special layered optics that are tuned to be nearly perfectly reflective, but only in a narrow range of frequencies.

> As you wrote they are limited by atmospheric friction...

That, and as the particles are charged, they repel each other and the beam disperses. Higher beam energies minimize this, but the effect still exists. The atmosphere might be the bigger problem, though.

I guess that theoretically they could just blast their way through the atmosphere, heating it until a line of near vaccuum is created. That'd take quite a bit of energy, though.