One subtle issue in learning words from reading is that with languages like English where there's ambiguity in how to pronounce a given spelling, sometimes you might not guess the correct pronunciation for words you learned. This isn't specific to learning non-native languages either; growing up I read a lot and learned most of my advanced vocabulary that way, so as a teenager I would often find out that I didn't pronounce a word correctly because I had only ever seen it written down before.