The legacy flight model is to allow imported FSX aircraft to kind of work. It changes a lot of things about the flight model, which aircraft are not tested against (both the stock aircraft and add-ons). As Asobo further tweaks the moden flight model aircraft for things like pitch sensitivity, etc, it seems to be causing even more problems in the legacy mode.
I have an open Wishlist item that people can vote on to change how this feature is presented/made available to reduce confusion - Move 'legacy' flight model option out of UI and into aircraft XML or .cfg, or label more clearly what it is for