Yes, it's true that reading and writing at an adult level requires knowing at least the "common" ones, about two to three thousand Kanji characters.

However, Hiragana and Katakana are not just for children or students. They form a significant portion of everyday text, and in fact necessary for constructing sentences properly in Japanese. For example, many/most grammatical particles like "of", "in", "to", "from", are written in Hiragana. And foreign/imported words are usually in Katakana.

Regarding the ease of pronunciation - I think the grandparent comment was talking about the sounds of the language, not about memorizing how to read and pronounce the text.

Japanese speech is composed of a fairly small set of simple sounds, similar to Spanish. Having taught both Japanese and English, I would say the latter has many more tricky sounds - like th in "the", or ar in arm (especially how Americans say it). In contrast, if one knows the sounds of each Hiragana (consonant + vowel combination), they cover almost all of Japanese speech.

You're on the right track. Individual words in Japanese can be represented in three ways:

1) Pure hiragana 2) Pure Kanji 3) A mixture of Kanji + hiragana

#3 comes into picture especially when conjugating verbs. As the base verb is written in Kanji but the various forms are tacked on in hiragana.

The important bit : The kanji symbol essentially just compresses the hiragana into a symbol, but all words that contain kanji can be uncompressed back to hiragana.

Japanese is definitely hard to learn, but it is not hard to pronounce. The hardness comes from the fact that the Kanji can have multiple 'readings' depending on the context.

Source: Studying/struggling with Japanese for the last 5 years.

I'd say Japanese is even more difficult than Chinese - at least with Chinese the majority of characters are predictably one syllable with the same pronunciation regardless of context; in Japanese you get entirely different pronunciations depending on the context, with a ton of exceptions.

That said the actual sounds are few and really easy to vocalize (if you ignore pitch accent). Adults do use kana for certain words and in between words, but it's true that you do need to know a lot of kanji to read things fluently.