Liquid metal is very conductive. When applied incorrectly, it can cause short circuits in your system. Considering the size of components in mainboards, it only takes a very small amount of liquid metal to bridge connections and cause a short circuit. Also, liquid metal can get into very small gaps like under memory chips and GPU cores. It is very hard to clean.
There are already plenty of cases of motherboards and GPU's damaged or rendered completely unusable because of leaking liquid metal.
I strongly suggest you just use thermal paste. Look for good quality thermal paste and thermal pads.