Pros
Great core business. Mainframes are still a growth area, 30 years after they were declared dead. Huge customers - enormous insurance companies, banks, governments, industrial giants that are not going away. No layoffs in either the Great Recession or the pandemic. You will not find a steadier job in the software industry.
Cons
Working on these ancient products can be tedious; knowledgeable people are getting old, so they hire young staff, who get bored and leave. Management loves to talk about "transparency," but hiring and tooling decisions are a black box, rank-and-file aren't consulted about anything. Too many acquisitions - a year or two after declaring "we won't do any more for awhile," the company makes its biggest acquisition ever. Dealing with IBM is a pain - clueless, logic-free decisions handed down constantly, and we just have to deal with them.