The new Dell XPS 13 9360 here is very similar to my previous XPS 13. Just bunch of small Torx screws on the bottom side holding the base together. Remove those (fewer of them in the newer model) and the guts are readily accessible. The SSD and WiFi card are the only real upgrades possible to it, and it's not a complete hardship to get at them.

I don't know what other brands/models are like though.

Our Precision M6400 beasts are more old-school, with the two screws (permanently removed) to access the slide-out 2.5" drives, and separate hatches for memory and Wifi. Very handy, for sure, especially the slide out drives. On the new model I've switched to USB3-UASP adapters for booting from other drives, which is just about as simple.

Really, what I miss on the Ultrabooks is the ExpressCard slot that the old beasts have. I used it to test drives and drivers for various native PCIe SATA controllers, and even stuck a USB3 expansion card in there (no USB3 on stock machine).

Cheers