At most I've spent a day to get some quirks of new hardware to work, but all of the 3 upgrades I've done throughout the years has basically worked directly.
And with Clover bootloader, it's even easier than it's ever been.
The key is to pay attention to what hardware to use, the tonymacx86 buyers guide is great for the most part but sometime the recommendations are too early so you might have to live with a few quirks like USB testing and limiting.
Right now I'm using an Gigabyte Z170-HD3, Intel i5 6600K, 32GB DDR4, 1x M.2, 6x SSDs and 2x HDDs and Nvidia 1080 Ti without a hitch.
I'm even using MacOS on my new HP Envy 13 without any issues.