This is a variation on the neoclassic buttercream, which is soft but still firm enough to pipe shells or stars or rosettes with as long as it doesn’t get too warm. I don’t think you could pipe roses with it.
A few things to check:
Is the syrup bubbling thickly all over (not just a little boiling in the middle)?
You can beat the eggs for as long as five minutes on high before adding the hot syrup.
Are you getting all the syrup into the eggs? Scraping the pan (or measure)?
Is the egg/syrup mixture cooled to room temp before you add the butter?
Is the butter about 65-70F when you make the buttercream?
Also, after the buttercream sits for a while it softens, which is lovely to eat but doesn’t work if you’re still trying to frost/pipe with it. It needs to be rebeaten to firm up again.