You Want Clean Software (Stock Android)
The HTC U12+ runs the latest Android 8 Oreo, and it's even upgradable to the Android P beta (according to HTC). However, the software (HTC Sense) they use to cover stock Android is something some people love, tolerate, or absolutely hate.
It's known as a skin, and it completely changes how Android looks, feels, and works. I don't mind some of the changes, and HTC actually has some extremely helpful software controls. That said, a lot of people try an HTC device and don't enjoy the customized software experience.
If you're looking for a clean, fast, smooth Android experience get the Pixel 2 XL, wait for the Pixel 3, or even get the OnePlus 6. You won't find that on the HTC U12+. Sometimes it just feels too dated, bloated, and overdone.