In our case: 1) slow and long transpilation times, 2) although not that common these days but missing typings used to cause some head scratch 3) overly permissive/overly restrictive type definitions 4) wrongly configured sourcemaps 5) unreadable type definitions/declarations... The list is long but these are off the top of my head

Getting a configuration together that consumes all your dependencies and generates code without crapping a ton of meaningless warnings about duplicate type definitions in its own type mappings for runtime types is non-trivial and requires setting more than a few non-default compiler options, none of which are named anything that means anything.

I still don't know how it's possible to safely, cleanly make a library that other projects can consume comfortably with just TypeScript.