I exclusively use FBC/IMBC/SMBC (plus cooked custard based frostings). I think vanilla is the trickiest because the butter does stand out. I use a generous amount of vanilla bean paste and the most wonderful homemade vanilla extract. Together, it is really good, but my least used flavor. The beauty of all three of these is their ability to take on flavorings. When flavored properly and paired with a great cake, the flavors meld together and create an overall great dessert. It is not overly sweet, but in my experience, those customers who like ABC do not compare it to ABC, it's just a totally different taste experience. Except for the vanilla, not a trace of butter flavor remains in my buttercreams. I have honestly never had someone dislike these buttercreams.
Be sure to use the best flavorings because it will ruin the product. Even some pure extracts have an odd taste and cheaper chocolate will make it just ok.
My favorites are spirits and liqueurs... Godiva, Frangelico, Chambord, Kahlua, Bailey's, Advocat, and my greatest buttercream accomplishment, Absinthe. It actually tastes incredible.
I use muscovado sugar for caramel, fine chocolate, gourmet jams, fruit purees, and so much more. In each case, you taste the flavor.