CA/PA of the teams you test against should be varying, cause some tactics are based on counter attacking, others on possession etc etc. A lot of tactics are great against good opposition, but has problems when teams park the bus and go long balls. If all teams you play against are around the same quality, they will all play pretty much equal. The formation isn't what's most important.
Perhaps you could try to put the legends team in a natural environment, a normal league? So that it will face teams of various strength. Me, Andrichband and Dbotiger are testing in England, perhaps you could test in another big league? Like Spain, Italy etc? Or maybe combine teams from the big leagues? An awesome idea: Take one big team, one middle team and one lower team from the big leagues in the world, and put them together in one league. Now THAT would be a great testing environment I think
I wish I had the skills to make such a database myself