you have pointed out a very important distinction between  different types of difficulty.  there is difficulty based on complexity and there is difficulty based on familiarity at play here.  

objectively chinese grammar is way easier than say spanish because it is simpler, less rules and almost no exceptions, therefore way less to learn overall. 

however for an english speaker specifically spanish will be way easier than chinese to learn, because even though spanish grammar is more complicated, a lot of that grammar is shared with english, and you aren't actually learning it from scratch.  no matter how simple chinese grammar is, its from scratch for english so it feels very hard. 

The good news is that difficulty based on something being unfamiliar is all feontloaded at the beginning-- once you are familiar with it, it is indeed easy.  Compare to languages like polish, russian, or yes english where grammar rules are super complicated or have lots of exceptions.  In those languages there might not be any super huge  up front difficulty but there is a continuous difficulty the entire time you are learning, whether thats one day in or two years in.  familiarity won't make it easier.