Bullet Punch is not used as coverage, it's used as priority. And it really sucks.
252 Atk Pure Power Mega Medicham Bullet Punch vs. 4 HP / 0 Def (Ice Type) Greninja: 204-240 (71.3 - 83.9%) -- guaranteed 2HKO
Bullet Punch is supposed to be for picking off weakened opponents but they need to be REALLY weakened. Grenina, with its 72/67 bulk and even when Protean'd into Ice, can easily survive a Bullet Punch. And it does NOTHING against bulkier targets.
vs. 4 HP / 0 Def Garchomp: 76-90 (21.2 - 25.1%)
vs. 4 HP / 0 Def Landorus: 80-95 (25 - 29.6%)
vs. 252 HP / 0 Def Clefable: 190-224 (48.2 - 56.8%)
vs. 4 HP / 0 Def Latios: 88-104 (29.1 - 34.4%)
Even with Fake Out+Bullet Punch combo, Medicham needs a LOT of prior damage for all of this stuff in order to RK. Really, if you want a RKer just use Greninja or something.
My favorite, and imo Cham's best set is still 3 attacks+Baton Pass, for a number of reasons:
-When 95% of OU's threats are covered with Zen Headbutt, Ice Punch and that nuclear High Jump Kick, the fourth slot is covering hardly anything. Fire Punch for Aegislash; Thunder Punch for Gyarados, Azumarill and Slowbro; Poison Jab for Clefable and Sylveon: really not much. Whatever you choose, something is going to be able to counter you anyway, and Baton Pass essentially covers everything that your other three attacks can't if you've built your team correctly.
-Medicham can't afford to predict wrong, especially when High Jump Kick is its primary STAB move. Imagine this: you're up against Rotom-W, who you can KO with HJK, but your opponent has Aegislash. You have Fire Punch, so it's basically a 50/50 prediction where if you guess wrong, Cham pretty much dies/becomes useless: use Fire Punch, predicting Aegislash to switch in, but instead they stay in with Rotom-W who burns you. Use HJK, predicting they'll stay in, but Aegislash comes in, you lose half your health, and Aegislash gets a free turn. Baton Pass allows Medicham to deal with these situations in a risk-free manner, by instead sending in a Rotom-W counter/Aegislash check.
-Baton Pass allows Medicham to safely MEvolve against slower targets that it can't KO.
-Baton Pass can give free turns to teammates when they're sent in on something that they can easily check
When Medicham already bulldozes through nearly every single defensive pokemon, save for a few, it's really easy to find Baton Pass recipients who can patch up the things Medicham can't handle, and take advantage of the free turn gained. Bisharp is especially good.