The biggest downside is the technology; VB6 was rightfully killed off by Microsoft 5 years ago, and MUMPS is a painful language to read. The architecture of the system hasn't significantly evolved from its original state, performance is lacking (the system slowed to a crawl in the middle of a class of 50 users), and the worst part is, the second-in-command considers the system to be "good enough" (which is never true, but that's unrelated).
While the pay is good, career advancement is practically non-existent, and they explicitly state "if you have a goal in mind, you probably don't belong here". The best one might hope for is "promotion" to Team Lead, which happens generally to those that regularly put in 50+ hours, and generally just means more work for the same pay.