Failing vocabulary - consequences?

Although you can sometimes guess the reading from the phonetic components, you should guess the reading from the meaning and the existing vocab repertoire in your brain. So that, it would be easier for exceptions and Kun reading compounds.

For example, 流行->fad->はやり、りゅうこう; instead of 流行->流+行->りゅう+こう->りゅうこう->fad.

This will also help not only with exceptions in Rendaku, but also exceptional reading.

Japanese is such a language that Kanji can say one thing, but Furigana can say the other thing, isn’t?

Yeah, I tell everyone I do EN->Kana, ignoring the Kanji. – on or [Userscript] Self-Study Quiz (But, I actually use Anki with Core 10k breakdown, though.)