The developer experience is terrible, it's a completely parallel world that diverged about 25 years ago from what we're used to, and it's not particularly good. Furthermore, it's so obscenely priced that it will be never used for anything new (and forget personal projects), so career-wise it's a dead-end and confines you to only ever work in existing legacy deployments.
The "corporate experience" for the lack of a better word is terrible too. Companies that run this have zero engineering culture (if they did they would've finished their migration off COBOL long ago) and developer positions have about the same amount of respect and political influence as the janitor. There are much better options pretty much anywhere else, so the only remaining people are too mediocre to get those better opportunities, so it's not a good environment to learn from either.
Migrating off COBOL (or any legacy system) is possible - after all, startups have built similar systems from scratch. The problem is that this requires a competent engineering team and you won't get that without a good engineering culture and embracing engineering as one of the key pillars of your business and giving it the resources and respect it deserves.