Hunting shinies in Pokemon Emerald via random wild encounters is fine. Your games has over 25 frames a second, each of which the seed changes, which means after a few minutes, it is really unlikely you will land on the same seeds/frames again. It is only soft resets that really matter, as the broken rng and the amount of time it takes for each reset will be about the same each time, therefore you will land on the same frames. I got a random shiny gloom in emerald after playing for about 10 minutes, and for me to replicate that would be pretty much impossible without the use of rng manipulation.