He doesn't construct points very well, he's generally either entirely defensive or all out attack. And even when he's all out attack he finds it hard to hit winners and on clay you need to be able to hit through opponents.
I'd say it's as difficult to hit through on most of today's HCs like AO, USO, and Miami, and Andy does well there.
But I have one theory about why there are well defending players like Andy who are clay mugs but great on slow HCs. It's easier to move on solid HC surface than on clay. That's also the reason why slow HCs, despite being a bit faster, playe effectively slower than clay.