Scales are a total game changer, right? A good pistachio buttercream is just ground nuts in a basic American buttercream. The real trick is using a high quality paste, not just extract. Have you tried browning the butter first? It adds a deeper, nutty flavor that works so well with pistachio.
Oh man, browning the butter is such a good call. It makes all the difference with nut flavors. I totally get where @gracem32 is coming from on the price, but a decent scale really does help you nail the ratios every single time, especially for something finicky like grinding your own nuts. Cheap extract can taste so fake and bitter compared to a good paste.