Magcargo has an abysmal HP stat and lacks Levitate, all traits that i mentioned in my post. If it had a decent HP stat (80 or 90) and Levitate it would be at least usable in UU and RU imo. A Fire / Rock type needs Levitate in order to be viable in anything outside of NU, as two 4x weaknesses is too much to handle for any Pokemon, even more for a Pokemon weak to SR.
And while the water weakness is a big downside, it is not that important, as most Water-type moves are carried by Water-type Pokemon which you have no business dealing with anyway.
Part of the OU QC team, message
me for a check!