It makes your watch look great! There are some major downsides however:
Silicone oil can make the rubber shock resistant components in certain gshock models swell which is fine till you need to replace the battery then the case won't go back together.
In cheaper, less waterproof models oil has a tendency to leak out. My Casio royale leaks which makes it always feel a bit greasy.
It can be quite hard to accomplish a bubble free hydromod unless you have a vacuum chamber.
It makes your watch look great! There are some major downsides however: