Ruby laser has following drawbacks or disadvantages:
1. As the terminus of laser action is the ground state, it is difficult to maintain the population inversion. This fact results in ruby laser’s low efficiency.
2. The ruby laser requires high power pumping source.
3. The laser output is not continuous but occurs in the form of pulses of microsecond duration.
4. The defects due to crystalline imperfection are also present in ruby laser.
Reference: This article is referred from my authored book “optics and lasers” having ISBN 81-272-2948-2. In case of any doubt, post in the comment section.