U guys are so funny lol
grab and usmash both come out frame 7. The reason you might think usmash comes out frame 8 is because of the jump cancel, which means a running usmash is 8 frames (but still the same frames as a JC grab).
[I was going to quote magus here, but my PM box must have gotten purged last fall somehow, so i no longer have his wise words; instead i will paraphrase]
Drill to usmash does work occasionally. It depends on a few factors, mostly weight of the character and when the last hitbox hit. I'm not really sure how weight factors in, though i assume it will modify the hitstun by a frame or two. The real tricky thing is hitting with the hitbox and then the next frame landing. If you didn't know already, fox's drill works as a series of hitboxes that have small gaps in between (Hit: 5-6, 8-9, 11-12, 14-15, 17-18, 20-21, 23-24). By hitting with a hitbox and then immediately landing you maximize frame advantage.
The reason this issue doesn't come up as much for drill->grab is because of how shields and dodges work. For a buffered spot dodge, there is an additional 2 frames before you are invincible and for a buffered roll an additional 4 frames. This gives a bit of room for error when grabbing, while if you usmashed in that case it would simply hit the shield.