Svenne,
I do know some weak spots of BIAB, but I do not consider specifically this as one. PG gives a basic set of sounds, even with the cheapest tier of BIAB....

Consider that Sonic by itself cost $250 (when not on sale)
BIAB or NOT BIAB Sonic IS the best GM (software) synth that would do program changes out of the box with possibility to swap instruments from internal library, which has decent soundest and highly tweakable. Unless you can prove me wrong.

Question is this. Would you pay even half of the Sonic price extra to get similar in BIAB? I doubt 90% of users will....So if you are looking for advanced software GM module, I believe there are no alternatives to Sonic / Halion. Get the free Sonic, and try it. Think of it as a next generation of Bandstand... but much better.

P.S. If you decide to try it and run into issues, I am sure there are enough people here who have it, to help you setting it up. Once you set it up, you are golden smile