I just tried this for the first time and although it's very smooth and tasty it seems very "loose" - like it isn't sturdy enough to stay on the cake or hold up as a filling. Is this normal? Like I said it's the first time using it so I can only compare to my regular BC recipe of PS and butter.
smbc shouldn't be loose, it should feel light and airy, but very firm. Maybe you didn't add enough butter, or whip it long enough. Did the buttercream look like it was curdled before it came together? It's normal for it to look that way, just let it keep whipping and it will come together. I use smbc as a filling and frosting all the time and it holds up very well. I would try adding more butter first and then letting it whip until you see it come together. Let it whip a good 5 to 8 minutes on medium high speed.