I have ss’d for a number of years, converting a number of non-ss frames, had an SS inbred and now have a Genesis iO rigid SS (steel) frame.
The thing that appeals to me about singlespeeding is the simplicity of the bike that I can just get it filthy on XC training rides and then just quickly hose it down and it will ready again for the next ride.
I have never ridden a Whippet so can’t comment on specifics there but I would imagine that it might benefit from a bit more TLC to keep it running nicely although actual owners will give better feedback no doubt.
All I can say is the next bike I am thinking about getting is a Singular Swift which is a very similar construction to the iO being rigid steel but in a 29er config.