Relatively speaking. The writing system is incredibly broken with the amount of readings of each kanji and kanji combination (each reading borrowed from a different part of China at a different point in time, making it seriously messy). Really, compared to that, the grammar is really simple. I'm saying it as a speaker of a language with eight functioning grammatical cases, grammatical gender and God knows how many declensions, so that may be what makes me consider Japanese grammar really simple.