Overheating: A problem everywhere. The logistics of a post-scarcity society may have eliminated most material-supply problems; advanced ergtech may have made energy not a meaningful practical constraint; computer power may be an absolute bargain everywhere; but even with every fancy high-tech possibility up to and including neutrino-catalyzed cooling in play, the requirement for heat dissipation still enforces speed constraints on nanotechnology, density limits on computronium, causes problems for guns firing on full-auto, prevents stealth in space, and requires most heavy industry to be kept at least in orbit even if its customers are downwell. Damn you, thermodynamics!
Frickin’ Laser Beams: Invisible. Recoilless. Travel at the speed of light. In short, just like actual frickin’ laser beams.
(See Energy Weapons and Kinetic Weapons Are Just Better for where they are used, and why they’re not usually considered the primary weapon set in the Eldraeverse, which I didn’t particularly feel like repeating right here.)
As a side note of things mentioned elsewhere on the defensive side, the chief purpose of lasers as ship-to-ship weapons is for pumping heat into the enemy. The defense against this is twofold: one, thermal superconductor (or, previously to the invention of this particular piece of exotic material, mere thermal-very-good-conductor-convector-etc.) plating to avoid localized hot spots heating up and exploding, and lots of goo of very high thermal heat capacity to dump heat into and then pump overboard. This doesn’t stop you from eventually having to heave to, extend your radiators, and quit the fight, but it does slow down getting to that point rather a lot. Similar plating to the former used in personal armor makes personal laser-arms, while not unuseful on the battlefield, certainly not the most useful thing which you can carry.
tl;dr They have their place in combat, but they’re not magically supreme on the field, and indeed are unlikely to be your primary weapons.
Also, while it would certainly be technologically possible to attach them to cyborg sharks, so far as I know, it’s never actually been done.
Instant Cooldown: No, no, no. Thermodynamics does not work that way, and even with thermal superconductors, neutrino pumps, and black holes, thermodynamics still doesn’t work that way. Stay out of the red zone.
(Also, bearing in mind the proclivities of the manufacturers of much of this equipment, the red line on the dial is where it will explode, not 10% before where it will explode on the assumption that many of the operators will be gung-ho idiots. Yes, there’s a safety margin to cover manufacturing variances, and suchlike. There is not one to protect you from yourself, as lying to you for your own good is not in their paradigm, belike.)