Possible they found serious problems with the first model and have specs that indicate the next model hasn’t done enough to address the issues...
Yeah, they figure the Surface Book 2 is basically just an upgrade, not an overhaul, of the product.
They claim their "not recommended" rating is based on their evidence that 25% of Surface customers reported serious hardware-related problems with the systems -- breakage, dead, hangs, random shutdowns, crashes, etc. -- within the first two years of ownership. Microsoft's response was that Consumer Reports didn't appreciate the product's appeal to the customers.
Personally I think Microsoft's design is based on the assumption that folks will want to buy a new one every few years. It's not all that different from Apple's philosophy, actually. Apple makes their money off the hardware sales, so they have to move physical product on a regular basis. Microsoft tried the same thing with Surface.