In my experience, scaling a large project in a dynamically typed language with 100% unit test coverage just gets you production outages with all unit tests passing. A reasonable threshold is probably closer to 70-90% _plus integration tests_.
Hopefully you also picked a language where a bigco has created tooling that lets you incrementally add static types -- TypeScript, Closure Compiler, Python typing and/or pytype, etc.
Lol, this is so true. A project that I had joined (late) some years ago had 98%! code coverage, full sonar/fortify code compliance and was an utter piece of garbage that had regular outages in production.
It taught me a couple of lessons:
1. Integration and System Tests are FAR more valuable than unit tests. By several orders of magnitude.
2. Performance Tests are vital. This is actually more true in a cloud world. There is only so much hardware you can throw at a problem before your costs are un-feasible.
Hopefully you also picked a language where a bigco has created tooling that lets you incrementally add static types -- TypeScript, Closure Compiler, Python typing and/or pytype, etc.