Kyurem- B can hold Life Orb and Choice Band, Mega Garchomp doesn't have that luxury.
I hate how people tend to downplay the actual benefits of the Mega Stones.
Imma take the 3 (offensive) items most people use on Kyubee and compare what they give to Kyubee to what already M Chomp already has:
- Band: welp, i guess i cant argue you 2hko the entire game. can I? then you realize that most of the times youre just clicking outrage, much like CB terrakion with CC, not using Dclaw or Fusion Bolt since they actually defeat the purpose of a Choice Band: sheer power, but with the problem there that its a move so easily taken advantage of its not even funny, 95 speed is sub par at best for something that is supposed to sweep, and with that, almost the entire offensive side of the meta can KO after locking into outrage. Comparably, yeah sure, chomp is hitting 0.33x (dont quote me on this, just fast calculating) weaker, but you keep wallbreaking capabilities due to amazing complementary (and not fucking redundant) STABs and no pressure due to hazards, something caused by any choice item.
- LO: i guess somebody here is forgetting Kyubee is vulnerable to all hazards, weak to the most important, and has an Abomasnow-level defensive typing, LO recoil is, in my honest opinion, a liability to something with such bulk. Hazards in itself take 1/4 to more from Kyub's lifespan, and it is supposed to be a wallbreaker in the level of M Garchomp, who not only resists rocks but has much better STABs (im just not stressing this enough) to hit through defensive cores. The boost in power isnt worth the recoil taking your already scarce chances to do your job well.
- Scarf: tbh, this is the only item i would ever consider running on Kyub when comparing him to Chomp, since its something the latter cannot actually do, but we all know the problems with Scarf Kyurem , dont we? imma mention them just for the sake of it: its weak, and its weak to hazards. Well, as a general statement, all scarfers are relatively weak, so thats a moot point i guess, but the weakness to hazards is a killer to what is supposed to be a revenge killer. Still, i wont deny its better off taking this job than chomp is, no doubt about that.
I would run M Chomp any day over Kyub, to me its a no brainer, Band and LO have horrible setbacks to a poke with a the traits of Kyurem, and Scarf has its issues on itself. And the ''it takes up your mega slot'' argument is really starting to get me upset, if im gonna look for excuses not to use a pokemon that could fit here perfectly to use another pokemon that will probably need other types of support to work well then wtf?