I use Multicannons alot an try to always have atleast two of them with me. Multicannons are not supposed to break down the hull but due to their good armor piercing stats, they are excellent for destroying modules.
For example, two multicannons can (if you can keep the good impact angle) destroy a Python faster than a Vulture if you target the reactor. Cannons are even better for that. But in order to archieve that, gimballed versions are a must-have because otherwise you have a really hard time hitting the modules which results in a disadvantage -> flares.
However, in my FDL I removed the gimballed multicannons and have now four C2F beam lasers (oooooh, Pythons will love me) and one C4G Cannon. The cannon is excellent for taking down modules and almost one-hit every small ship without shields.
A full multicannon loadout can be effective as well due to the low energy draw, your can have a mobile shield tank whith decent firepower. However, you quickly run out of ammo. Some kind of additional ammo storage for the interior of the ship would be nice.
One major disadvantage of them are that they are not "hitscan" (laser-like. No projectiles to be dodged). This means that smaller ships have a huge advantage against multicannons due to their small hitbox. Because of that, they can 1. dodge the shots better (more agile) and are not hit by multicannons because they actually have some spread which results in some misses.
Conclusion: Multicannons: Good when using more than 1, good against modules, medium against hull, medium against shields, good heat and pwoer 'price', low-medium damage overall, harder to hit small ships.
Hope that helps :]