My IMBC, FBC, and SMBC are very stiff. I actually add teaspoons of hot water when I want a more billowy buttercream.
GBC is not as stiff.
I'll leave the SMBC explanation to FromScratch because I don't have as much experience with it.
In IMBC, two things contribute to the stiffness. First, you are turning the whites into a confection by introducing the 245 degree sugar syrup. When you try to free the meringue from the whisk, you get the idea just how strong the concoction actually is.
Next, you are adding butter. You can now manipulate the stiffness/softness by manipulating the temperature. Some choose to smooth cold IMBC with warm knives and bench scrapers. Room temp, maybe 72 degrees makes for easy piping that retains its shape even though it can get warmer.
My advice on IMBC is to look up Warren Brown's tutorial on his Cake Love site and make sure you have a calibrated candy thermometer. This is my recipe of choice and many others. It is balanced and he takes the temp higher, resulting in a more stable buttercream.
There are many recipes out there, some with more and less sugar/butter. You will have to pick the one that you like. Some pointers... cheap butter is just more water, salted butter will make an overwhelming butter taste. In these buttercreams, especially IMBC and SMBC, you have a clean palate. On other words, they provide the medium, but the flavoring takes center stage. What you choose to flavor it with will determine the final taste. The higher the quality of the addition, or more pure, the better the taste. But that last statement is just my opinion. In these buttercreams, the chemicals of artificial additives cannot hide. You have already made an incredible buttercream. Just be aware of how the flavoring will affect the final taste.
I have found that grocery store extracts, even when pure, give off a candy-like sweet taste. Higher quality extracts, liqueurs, purees, fruits, will give you a less sweet, more natural taste.
Sometimes your first try with one of the European buttercreams may not work out. But with help from the many threads on the subject, you will be successful.
I hope this helps. Susan