I think that there are some features which can be said to be objectively more difficult than others. For example, all other things being equal, a language that

I think that there are some features which can be said to be objectively more difficult than others. For example, all other things being equal, a language that had a larger phonemic inventory than a second would be objectively harder. Now do note that many comparisons means that it is easier on one feature while being harder on another: for example, Chinese has fewer phonemes than English does, but Chinese is tonal whereas English is not, and this makes it difficult to say if Chinese is (at least in terms of phonetics) easier or harder than English. While it is probably possible to tease out some sense of an objective scale for the phonetics or orthography of a language, when it comes to the grammatical features of a language, I don't think there is a realistic hope of identifying any reasonable scale of easier/harder languages. What makes languages easier or harder with respect to grammar is going to be less driven by absolute metrics of the grammar has this difficult or easy feature and more by the relative metric of does it have grammatical features known or unknown to the listener.