What the docs say:
Official docs (AB Tasty, Flagship cache modules caveats): the real compromises of running feature flags at the cache level. The visitor ID and context must be computed at cache level with only cache-server information, so a visitor ID might be a hash of the IP and high-level visitor data like database attributes is not available for targeting. Cache granularity becomes the flag key/value combination per visitor instead of per URL, so misses happen more often. The module also consumes extra CPU and RAM on top of normal cache usage.

What works:
If you run AB Tasty Flagship at the cache layer: keep targeting rules to what the cache server can see (IP, user agent), database-backed attributes will not be there. Expect a different cache-hit profile since entries key off flag values per visitor. And budget extra CPU and RAM for the module when sizing the cache tier, it is not free. If targeting needs rich visitor data, do the decision server-side instead of at the edge cache.