>>31001070In addition to what the others have said, going from just above 100 base speed to just below is huge, Garchomp is lucky to be in its speed tier, and with no ways of boosting its speed up again (no choice scarf, no dragon dance) it's easier to beat.
It also loses a very useful ability in Rough Skin and gains a situational weather relying one. Hell, Sand Veil would probably even be better.