Norway is among top countries in the world for English L2 proficiency, but an American told me that he'd heard many Norwegians using present progressive incorrectly, e.g. saying "I'm living here" when it's supposed to be "I live here". On the other hand, I'm sure that a majority of Norwegians could pass a C2 English exam without much preparation. So the answer to your question depends on what you mean by "learnability".
If "learnability" is about learning to be indistinguishable from a native speaker, the presence of the progressive/non-progressive distinction is definitely an obstacle for speakers of languages that don't have it (e.g. Norwegian). However, most people don't have so high requirements when they learn a foreign language, and one could argue that occasional slips in the usage of progressive tense among L2 speakers don't make a language unlearnable.
The tendency to overuse present progressive by Norwegians is confirmed by a study of lower secondary school English exams in Finnmark, Norway (behind a paywall, unfortunately):
The clearest example of generalisation on the morphological level is the overuse of the present progressive. This is a morpheme that is acquired early and learners seem to think that it is a salient element in English, although research shows that this verb form is actually not frequently used (Kennedy, 1992). Below are only a few of the examples found in the material:
(12) If we had not videos in Norway, willing we wish that we had that
So don't be afraid when you looking at videos
The next day we used spending on Londons many parks
I have living in new york in 5 years
He thought it was very funny and didn't listening when I told him to stop
Clearly, the learners have understood neither the form nor the meaning of the present progressive, as the auxiliary to be is left out or replaced by another auxiliary, and the participle is used in various contexts.
My intuition is that progressive tenses aren't very difficult to use correctly, in most cases. My native language doesn't mark the progressive aspect, but it never felt like something difficult to understand, and apart from occasional slips, I can use it right most of the time. It is definitely much easier to learn than, for example, the usage of definite and indefinite articles (for speakers of languages that don't have them, such as my native Polish).