It has a poor special movepool with only 5 different special move types. It really only has 3 use able types, as normal and one dark aren't amazing(the best normal is swift, and best dark is snarl if you wanted to know). Basically, if you're special, you would be only using grass knot(only grass move) aura sphere/focus blast, and thunderbolt/thunder for rain. It wouldn't really be doing much, although it can be useable. In rain, thunder is nice, and it can be a surprise because most are physical. So, although it's special attack stat is actually decent, it's movepool in terms of types is disappointing, and makes it worse than physical although it can be useable, just that you need the right conditions, but it really has no advantage over a physical one.
Oh yeah, and for mixed also a no. Focus miss is worse than close combat, as is aura sphere. Thunderbolt(the non rain option) is worse than plasma fists. The only special moves you might see are volt switch and grass knot. Grass knot can be largely unreliable, due to being a weight based move. Likely the largest ground threat in ou at least, is excadrill, who isnt even weak to grass. It also largely helps against hippowdown, which would likely be brought out to wall physical moves from the common physical zeraoras, as well as hit back with an earthquake. I should also add that grass knot can be helpful, but it isn't worth using a mixed set or a special or mixed set for it, better just to switch. Volt Switch would kinda make a mixed set okay, but as I said, it's as pivot move and isn't really designed to do damage; it's not worth it to invest in special attack just for a pivot move.