Does only the “Thug” variant combine with the “Sneak Attack Fighter” Variant?

Unearthed Arcana lists the two Fighter variants "Thug" and what is commonly referred to as "Sneak Attack Fighter". I have been reading in forums here and there that you can combine the "Sneak Attack Fighter" with other Fighter variants, but the Unearthed Arcana entry for the "Sneak Attack Fighter" reads:

This variant can also be combined with the thug variant.

As far as I know variants are not mixed such as archetypes might do in Pathfinder. Am I correct to assume that by raw you do not mix class variants?