Gyarados is x4 electric weak ! He can't be OU for sure !
Volcarona is x4 Rock Weak ! It can't be OU as well !
Salamence is x4 Ice Weak ! Sure as hell out of OU !
On more serious note I don't think anyone serious says that if something is x4 weak to something, it means it's not viable (especially if it's the only x4 weakness). Grass/Fighting as a mentioned (and not only me) is great defensive typing, especially for physical walling, for which Chesnaught has stats to do. You can't ignore resists to -Rock, -Grass, -Water, -Dark, -Earth, etc. which are great in a metagame where Earthquakes, Stone Edges, Crunches, Waterfalls are flying technically everywhere. Your ability to counter/check Excadrill (huge), Landorus-T, Landorus (huge), Tyranitar (just watch out on Fire Blasts and Ice beams), Terrakion, Gliscor, Gengar (this ability :D), Keldeo (fun fact is that he can't 2HKO you by any move outside of Icy Wind and if you just run 252 HP/252 Def with Careful or other SpD boosting nature [You don't need ANY EVs in SpD, just nature boost] you avoid 2HKO from Specs Hydro Pump with SR up 100% of time and Sacred Sword hit you on your stronger defense), Tyrantrum, Hippowdon, Gyarados (even with Bounce, as you may just use Spiky Shield on a turn when Bounce ends charging up] and few other really strong OU Pokemon IMO give you 100% OU solid spot (even if it low OU). Look at those Pokemon he check/counter again - we talk about OU strongest behemots here. IMO don't underestimate this guy, especially in a metagame where stall/bulky offense/balanced teams are the most effective strategies to pull off. And he's not only limited to defensive sets - His base attack is good for example to use Choice Band. Or more offensive Bulk Up, so you may skyrocket your damage output. Or more defensive Bulk Up. Or even Swords Dance. He's much more versatile than you may expect and should find a solid spot on many teams, even those more offensive ones.