I do not think it is possible.
You should always compare a "source language" with a "target language".
E.g. how can you estimate the complexity of English pronunciation without specifying a "source language"?
Phonetic systems can be very different among languages. The English pronunciation is more or less easy for German, Dutch, Swedish-speakers, it is quite difficult for Slavic people (including Russians), and it is extremely difficult for Japanese or Thai speakers...
So, you cannot just say "the English pronunciation is easy" or "the English pronunciation is hard". You just have to specify "for who".
As to the grammar:
English has indefinite and definite articles which are difficult to grasp for those who do not have them in their mother tongues.
English has a complicated system of verb tenses (with all those progressive, perfect, perfect progressive forms). Many languages do not have such a system.
English has a complicated system of modal verbs.
English is known for the extensive usage of phrasal verbs, which are difficult to memorize for many non-natives.
Some syntax structures in English (with infinitive, gerund constructions etc.) are difficult to grasp for foreigners.
Irregular verbs also matter.
Again, everything has to be considered in comparison.
You can also check the link:
Difficulty of learning languages - Wikipedia, the free encyclopedia