Why would we simulate baseband rather than passband
One very obvious advantage of baseband simulation of communication systems is the low sampling rate it requires.

Considering a narrowband communication system having a few MHz of bandwidth to a few GHz of carrier frequency, it would be much easier (in terms of computation) to simulate it in the baseband sampling rate.

The reliability and the quality of a baseband simulation depends mostly on how accurate and realistic your mathematical modeling of the actual physical channel and the modulation system using it.