Assertions in code cut both ways. Sometimes they can be great; telling you exactly which assumed invariant is violated. However, that doesn't tell you where or how the invariant was violated.

Sometimes assertions are just crutches for lazy programming. Instead of handling of a very valid (corner) case, some people just assert that it doesn't happen. Lo and behold, years later, it does happen. And those years later, the context is completely lost. How easy is it to handle the corner case now? Hard. In this situation, instead of simply asserting that it doesn't happen, it might be warranted to just assume that it _can_ happen and handle the case. Then, followed up with coverage testing, one needs to (actually try hard) come up with an input that triggers it.

Assertions can also have bugs. The worst is trying to figure out which assertions are really valuable and which aren't.

Shotgun-spraying assertions in the code is not a good strategy in general.