I've seen just as many FP projects strain under technical debt as I have OO projects. Technical debt is a product of the developers, not the language. And FP developers are just as imperfect as OO developers.
However it is a significant improvement on allowing one to express the constraints in the type system preventing large classes of bugs and towards providing proper abstractions.
Let's not throw the baby out with the bathwater. No one is claiming FP developers are "better" than OO developers. In fact why would anyone ever classify themselves as one or the other? Instead we should use the best tools for the job.
We primarily write libraries, and our 'applications' are more or less throw away code because of how small they end up being (because they can leverage many of our libraries). Testing is easier in functional land (for various reasons), which means regressions are much harder to introduce, which means refactoring can happen faster, which means less technical debt.