In my experience, being difficult to test is a sign that a refactor is needed anyways. Difficulty testing also creates a higher barrier to adding new features.

In my experience, being difficult to test is a sign that a refactor is needed anyways. Difficulty testing also creates a higher barrier to adding new features. If you can't test something, it reduces confidence that your change won't break it. If it's an extremely performance-sensitive component, then you'd obviously want to be very cautious about big changes. However, that's usually not the case.