The problem with change is that both nature and us are adapted to CURRENT situation and all the land human has claimed acts as barrier for nature to adapt. And some things are slow to adapt. There is order of magnitude difference between negative and positive effects partially because all things previously mentioned. Humans have buildings and roads and all kinds of infrastructure in places which are optimal for current climate. Also too much heat is inherently bad. Too much cold is inherently bad, unless you had thousands of years adaptation for it. Alaska maybe better off once adaptation has happened in 300 years or so, while continental United States is inherently worser place, PEOPLE have adapted to current optimums and live and have build things based on that, 99% are worse and 1% are better, and that 1% is only better after they have survived immediate effects of change like permafrost melting caused problems.
Subject an open plains farmland in area to once a decade rains of rainforest and top soils moves away while having same thing in rainforest is not a problem. Now you got the real picture. Its all about mismatch between situation on the ground and whats in climate. On planetary scale it probably increases deserts and makes certain cold climates more habitable EVENTUALLY, while extincting things that depend on extreme cold. Maybe Greenland becomes agricultural paradise eventually once they have build soil there. But before that has happened you would of lost huge amounts of agricultural land elsewhere. And all that ice melting would mean New York would be below sea level long before Greenland would become agcricultural land. Of course if New York would be tens of miles inland then there would be no problem, but it is there right besides sea. The time difference between gaining and loosing is big problem. You cannot move food from future to present.
So yes, Canada, Alaska and Siberia might become more habitable. On the other hand most of the world would become either worse or LOT worse.
And in WORST case climate change we would get another habitable continent, while loosing bigger area to sea level rise AND deserts BEFORE it becomes habitable.