Look at Bjorn's videos on YouTube. He's from Norway and has done extensive coverage of the good and the bad with Tesla. He had a fairly early Model S with the sunroof and has done at least one video about it and it's limitations. He did put some foam insulation up there in the winter to help keep in the heat a little better.
He has a Model X now, but look at his archive and look for videos where he talks about the roof.
Personally I prefer the solid roof and got one on my S, but it's no longer an option on the S and I don't think the solid roof will be an option on the 3 either, which I consider a flaw. But while there will be some extra heat loss in the winter from the glass roof, it can be mitigated like Bjorn did with his Model S.
I did have a loaner with a sunroof last winter and it wasn't as bad as I thought it would be. The glass is fairly well insulated and the extra light from above wasn't annoying like I thought it would be.