i've had no issue getting my Z390 Gigabyte Aorus Master with 9900k to 4.9 (on all cores) with pretty decent temps - and tomorrow i work on 5.0
i'll agree the BIOS GUI is a little rudimentary but it still gets you there
here's a vid detailing each page of it's BIOS
https://www.youtube.com/watch?v=d8rY4TrcDXg&t=704s
i'll agree the Asrock has an easier to use BIOS interface, but then the latest BIOS i flashed bricked my Asrock Taichi motherboard - and researching that BIOS (which is what i should have done before flashing it) found 2 other posters that had had similiar issues with the P1.50 BIOS release - all that was within 5 days of the BIOS' release, then someone popped into my thread that had the same experience ie bricking his Taichi board with the next release P1.60. It looks like Asrock pulled that P1.50 off their website altogether
so if you go with an Asrock Taichi, stay with BIOS release no higher than P1.30
here's the thread if you'r interested
http://www.tomshardware.com/answers/id-3825393/heads-asrock-z390-taichi-motherboard-owners.html
here's a pretty decent thread on the very subject of which Z390 board - http://www.tomshardware.com/answers/id-3818069/buy-asus-z390-maximus-hero-gigabyte-z390-aorus-master.html