That feels like it might be a bit too strong, especially on the types of characters with tons of skills like an orc ranger or a lucky bard. I do agree that they feel like they need something extra, though.

Correct me if I'm wrong, but right now, they only up dice (aka theoretical training) and not advancement/skillchecks (aka practical training), right? Why not make it to where it applies a (modest-to-moderate-to-extensive based on BUC) amount of practical training too?

Maybe upgrade 1/3/5 skills, or a random number instead of all skills? I agree that upgrading all skills seems too powerful and reliable.

If your character has tons of skills - it is his own merit, he has honestly learned them, so getting more benefits (including from this potion) is his just reward. If he only gets to improve a single random skill (or any limited number of random skills) at once, he is penalized instead (since junk skills can be improved instead of something useful).
If it is deemed too powerful, the advance can be reduced a bit (i.e. instead of upgrading from "+1" to "+3d3" - only go to "+1d3"). Though only playtest can tell if it would really be that good, since the improvement is only to the potentials - so characters are still limited by skill practice and Learning score.

It could also increase the dice on all skills even if it doesn't actually increase all skills.