The banshee itself was not a bad product, but when compared to the competition at the time (TNT came out around the same time) it just couldn't quite match them.
Not taking into account glide support (because at that time everyone was moving towards the OpenGL and D3D standards anyway), it lacked a few important features such as full AGP support, 32-bit colour rendering (not real important at that time as performance hit was bad anyway), lacked support for textures larger than 256x256, and that it ran ludicriously hot compared to just about everything else all counted against it.
In a way it was a good attempt at trying to get Voodoo2 SLI performance into one card, but it didn't actually offer anything else other than that.