Problems associated with language endangerment:

The speakers of endangered language and linguists (or revitalization workers) might have different views on the revitalization and language itself. For instance, for a language community it might be enough to know some of the language, whereas linguists want to revitalize "the whole language". Thus, there are contradicting views on what are the goals of revitalization. In addition, speakers of one language community might have varying opinions and views on language and language revitalization within the community, making it more difficult to determining what are the goals and purposes of revitalization.

There are multiple factors related to language vitality and endangerment, not just the bare number of speakers. Other factors effecting the vitality are the domains/environments where the languages are spoken in, what are the different language variants, the age of speakers, transmission to next generations...

In addition, in general it would be important to pay attention how we linguists talk about endangered languages - for instance determining a language dead by using this word might be harmful, and language speakers might see the "status" and endangerment differently than linguists.

We also often talk about language endangerment, although what is endangered is much more than just language, it is the culture and knowledge that are lost too (for instance the kinship terms in some Australian communities).

ADD:

Mufwene, S. S. (2017). Language vitality: The weak theoretical underpinnings of what can be an exciting research area. Language 93(4), e202–e223.