not familiar with tachyons and just did a cursory look at it, what about tachyons really solved your problem?

The problem of locally scoped CSS vs shared styles across components sometimes.

With tachyons I still wrap up styles in components but I also get globally scoped CSS for consistency with fonts/spacing.

Honest question: how do tachyons differ from a (well-thought-out) collecion of less/sass mixins?

Tachyons differs in that it gives you consistent typography/spacing but without the added complexity of a build step.