Disabling Shot is a very useful ability, but you have to be smart about when you use it.
Sectoids and Sectoid Commanders all carry Plasma Pistols. Sectoid Commanders also carry an Alien Grenade. You will generally not have Disabling Shot until you have stopped facing Sectoids regularly, and the Mind Control ability of Commanders cannot be disabled.
Outsiders carry a Light Plasma Rifle and an Alien Grenade. Again, you typically won't have Disabling Shot until they stop appearing at all.
Thin Men and Floaters all carry Light Plasma Rifles. Both of these appear throughout the game, particularly Thin Men who show up often in Council Missions, so Disabling Shot is a handy tool against them.
Mutons carry Light Plasma Rifles for the first month after they appear; after that, they carry regular Plasma Rifles. They also carry one Alien Grenade each. Disabling Shot usually forces them to spend a turn reloading, which can buy you that extra turn needed to get in close for a kill or capture.
Heavy Mutons and Heavy Floaters all carry Heavy Plasmas, and a single Alien Grenade each. As with Mutons, Disabling Shot can be handy with capturing the last of a group of either type.
Cyberdiscs have their primary autocannon, and an Alien Grenade. Disabling Shot is extremely useful against them, as their cannon spells instant death for even mid-game squaddies.
Sectopods have a particle cannon, a turret, and a mortar. The turret is used only on reaction fire and cannot be disabled. The mortar requires at least one round of preparation before it fires. Disabling Shot is situational here - if they try to use their mortars instead of reloading, that could make things worse instead of better.
Drones have their beam. They can also repair other mechanical units instead. Disabling Shot is generally wasted on them, as their beams do so little damage.
Muton Berserkers have claw weapons strapped to their arms. This uses no ammo and cannot be disabled.
Ethereals, Chryssalids, and Zombies carry no weapons. They also cannot be disabled.