I think this trend should be followed responsibly. If you try to recreate this in HTML/CSS even without parallax, you end up with too many layers floating over each other that slow down your website.

On your second point — how hard it is to create similar UI in Sketch — I gave it a shot here — https://dribbble.com/shots/3296210-Flower-Power . You can do it with Vector graphics easily enough. But if you are using photos, be prepared for a ton of Photoshopping.