On the MBP, I experience semi-frequent freezes, kernel panics, overheating, unstable WiFi, inaccurate battery status reporting, scaling issues, occasional green tint, FS lock up etc. - it's still nice, but I wouldn't say it's more smooth than modern Linux with GNOME 3, in fact I've had much less problems with that setup than with my MBP.
Apple has engineers dedicated to building hardware and software that work together. They design and manufacture their own hardware. System76 doesn't. They pick out some components, and get them assembled elsewhere, and ship that out to you.
A better comparison would be Dell's XPS line, which at least has some number of people actively working on making sure they work well and are well supported, but still not to the same degree as Apple, and my understanding is they're generally told "make this work with Linux", as opposed to building a laptop specifically for linux.