I sold my 15" MBP and got a used SP3 and built my self a decent desktop for the price of a well speced SP4/mid-range Surface Book.
Sketching is great. My SP3 has no hiccups when sketching - the pen could be a bit better but that was improved with the SP4. CAD is a bit more of a toss up. It really depends on the complexity of your models and what you’re doing with them. Rendering and simulation will be really slow. But most standard modeling should be handled well.
I think it’s great for sharing idea though. The tablet makes it easier to have several people looking at something. The pen makes it easy to take a screen grab and put notes on top.