Wow... Ground doesn't play nice with a lot of other types when added to them lol. Mostly since the 3 weaknesses it has rarely line up with other types.
I'd say defensively it also pairs well with Fire, resisting 5 types and being X4 weak to Water and X2 to ground. Those types being common makes it hard.
It also pairs well with Fairy imo, getting an extra weakness to steel, but resisting 6 types (if we get a mon like that)