I recently wrote a guest blog post for the Ethereum Foundation comparing security assumptions underpinning four threshold signature schemes, including FROST. As a summary, the key difference between efficient two-round schemes (FROST and variants thereof) and three-round schemes is that the security of FROST reduces to the one-more discrete logarithm (OMDL) problem, whereas three-round schemes reduce to the discrete logarithm problem (DLP).
You can find more information in the post here.
