Principia was written during the naive Logicist era of philosophy of mathematics that couldn't foresee serious foundational decidability issues in logic like Godel's incompleteness theorems, or the Halting Problem. Formalism/Platonism and Constructivism are two streams that came out of Logicism as a way to fix logical issues, and they're (very roughly speaking) the philosophical basis of classical mathematics and constructive mathematics today.
The way formalists (mainstream mathematical community) dealt with the foundational issues was to study them very closely and precisely so that they can ignore it as much as possible. The philosophical justification is that even though a statement P is undecidable, ultimately speaking, within the universe of mathematical truth, it's either true or false and nothing else, even though we may not be able to construct a proof of either.
Constructivists on the other hand took the opposite approach, they equated mathematical truth with provability, therefore undecidable statements P are such that they're neither true nor false, constructively. This means Aristotle's law of excluded middle (for any statement P, P or (not P)) no longer holds and therefore constructivists had to rebuild mathematics from a different logical basis.
The issue with Principia is it doesn't know how to deal with issues like this, so the way it lays out mathematics no longer makes total sense, and its goals (mathematical program) are found to be impossible.
Note: this is an extreme oversimplification. I recommend Stanford Encyclopedia of Philosophy for a more detailed overview. E.g. https://plato.stanford.edu/entries/hilbert-program/
Nobody argues about the result of an addition because the computation is mechanistically verifiable. Same with statements that are properly formalized in logic. The goal was to have the same for all of mathematics. So incompleteness is not a problem per se -- even if it shook people so much at the time (because proof theory always work within a given system). Incompleteness is the battery ram that is used to break the walls of common sense.
If incompleteness isn't the killer of the Hilbert program, what is? The axiom of choice and the continuum hypothesis. Both lack any form of naturalness that would prevent any philosophical arguing. Worse, not accepting them also do. There is such a wealth of intuitionistically absurd results implied by these systems -- most famously, there is the joke that “The axiom of choice is obviously true, the well-ordering principle obviously false, and who can tell about Zorn's lemma?”, when these 3 statements are _logically_ equivalent. So, we're back to a mathematical form of epistemological anarchism; there is no universal axiomatic basis for doing mathematics; any justification for the use of one has to be found externally to mathematics.
I would add that there is/was a certain desire for categorical theories.
"In mathematical logic, a theory is categorical if it has exactly one model (up to isomorphism)."
(categorical is stronger than complete)