I don't think "unreliable" is the word I'd use. Astronomical events happen at times that can be calculated reasonable well. But when the linkage of those events to historical happenings to happenings on earth is speculative, then astronomical dating is pure bogosity.
For example, we can determine the time when Halley observed Halley's comet because we can couple the appearance of the comet to the historical event. Usually we cannot do that.