Trying to autogenerate biomes can be difficult. Average temperature and rainfall can only be generalized by latitude. Prevailing winds and ocean currents play a large part in determining those values. To my knowledge, FT does not take those variables into effect when calculating rainfall and temp. There are also other factors like soil type that affect the vegetation density of a location. That said, judicious use of FT's rainfall, and temp editors along coastlines and on the leeward side of mountain ranges and comparison to actual locations on earth that are at the same latitude and elevation yields pretty decent results.
Also, "plains" are essentially areas of low topographic relief and generally short vegetation. But if you look through examples of biomes of the world youll find great variation, for example in savannah and grassland biomes, you'll also see some that are more heavily forested than you'd expect.