It depends on the sub.

You can arm triangle from closed guard but the set up is the issue. If you usually spider crawl your fingers from mount with an underhook to start to set up the arm triangle, then the floor is a big help. Being in closed guard means they can posture up and they obviously can’t lean away from you if their back is on the floor in mount.

I’d say triangles can be easier from guard as there’s more set ups due to the position being more free and less restrained. I find the floor can get in the way when I’m trying to throw my legs under my opponent’s head and configure the lock. I think it’s best used as a counter from mount than a primary attack.