@gulios,
When using a Vega GPU it's advisable to install Lilu + WhatEverGreen which applies a few patches and ensures that the correct frame buffer is configured (usually "Kamarang")
Most Sapphire cards are based on the AMD reference designs as such they tend to be more compatible with MacOS which is why i favour them .... I have no experience of the Asus so cant really comment, however the 10.14.5 update resolved a lot of thermal issues (mainly fan control) with OEM Vega cards so it will most likely be ok but i would recommend you search the forums and find someone with a success story of using the "Asus Radeon RX Vega 64 ROG STRIX O8G Gaming" card before committing to one.
For more info on AMD GPU's on MacOS see the Compatibility Guide :-
Radeon Compatibility Guide ATI/AMD Graphics Cards Last Change: December 2nd, 2017 The "boot to black screen" issue as well as the broken sleep/wake have been fixed! Read below for details. 0. Introduction Most of you might have noticed that the Buyer's Guide doesn't list a single...
www.tonymacx86.com
However it might be worth holding fire for a month or so, AMD have recently released details of the new Navi cards (Radeon RX 5700 series) which go on sale July 7th ... and early support for them has already been seen in Apples AMD drivers for MacOS.
I don't think they are going to be as powerful as Radeon VII but they use a new architecture (RDNA) that may be better optimised on MacOS.
I would wait and see how they benchmark against Vega 64 before making a decision, also when they hit the market its likely that Vega Gen 1 cards will probably get cheeper.
Cheers
Jay