Official/Advised/Allowable maximum number of records (or RAM Table Size) in ETS / Mnesia RAM Table
mnesia is a wrapping layer of ETS and DETS tables. For a 32bit system a maximum value of a ETS table is about 3.5GB. For a 64bit system maximum value is much more, so technically it is limited by RAM available for OS. For DETS (and it is very stupid) limit is 2GB.
I don't have any benchmarks with other key-value storage systems, but remember, that ETS stores Erlang terms directly without any encoding/decoding.