Well.... the ROSAT doorway was on the guardian of Earth's lookout... which is on Earth. The ROSAT may not have been built to cater to aliens on Earth rather than the inhabitants on Earth.
Goku, as a kid, had a power level of less than 408 when he was in there and survived less than a month. Whatever biological advantages he has for being a Saiyan... there's no way those advantages are going to give him a better edge over someone, who by the very least in a conservative estimate, should be around 100K in power level. That's the way I see it. It doesn't seem that far off of a stretch.