Slaking isn't bad per se, but due to it's Truant ability and lack of an alternative/hidden ability, it is compromised despite its formidable offensive and defensive stats. Truant means that every second turn, Slaking "loafs" and this functions similarly to a recharge turn. Pokémon that know Protect, more common in competitive, can exploit this ability and alternate attacks on its Truant turn with Protect when it can attack, rendering it pretty much harmless. An opponent can take advantage of the inactive turn in other ways, for instance scoring a free switch/heal/boost. Given the strong likelihood of a Ghost type on competitive teams and Slaking's coup de grace being Giga Impact (for obvious reasons), a free switch seems feasible even on its good turn. One could argue that it can still hold its own when deployed late in battle against the right opponents, but generally it's a risk in singles. 2v2 and 3v3 is where it really shines, as you can employ strategies to alter its ability to a non-hindering one using moves like Skill Swap/Entrainment/Worry Seed or abilities like Mummy. Slaking is a beast when it can rampage uninterrupted.