There have been a few posts like this, and the discussion always seems to focus on plain JavaScript vs TypeScript. I'd like to see some talk about Flow vs TypeScript instead. We have what I'd call a medium-sized project (~30k loc) written with Flow and TypeScript's superior tooling is indeed attractive enough that we too have been thinking about making the switch.

Having used both, there are a few features in Flow that I'd miss. I think classes being nominal is a good approach and the ability to declare new nominal types is useful. Otherwise Flow's type system is structural, like TypeScript's. Function parameter type inference is also very nice feature, so I can make simple module- or function-local helper functions without needing to specify the parameter types explicitly. Having a syntax for specifying parameter variance allows for a more sound type system in some areas.

Performance has historically been pretty bad but it's been getting better with every release. The tooling has also seen some improvements after Facebook nuked their own Nuclide-editor project and moved on to endorsing Language Server Protocol and VSCode. It's still far, far behind TypeScript, though. It's also a bit worrying that the community seems to be getting smaller, not bigger, with posts like this popping up.