Someone seeing an unfamiliar kanji-based term in a Japanese technical document would often have little trouble reading it phonetically.
Furigana helps if one or more of the kanji used is rare (outside of the jōyō kanji), or when the word follows some common variation (guessing which is possible, but wastes the reader's time) or when the kanji spelling is arbitrarily assigned ("ateji"; impossible to guess).
E.g. suppose the reader sees 帰納. It is very likely they can read it correctly as kinō. The problem is not knowing what it means, that it refers to (mathematical) induction, or inductive reasoning. The dictionary lookup is trivial, though.
Basically, someone fluent in Japanese can read unfamiliar words like that most of the time. They will know a ton of words in which 帰 is ki and 納 is nō. In situations like that, it's not much different from someone encountering an unfamiliar word in English or Spanish text.
Furigana helps if one or more of the kanji used is rare (outside of the jōyō kanji), or when the word follows some common variation (guessing which is possible, but wastes the reader's time) or when the kanji spelling is arbitrarily assigned ("ateji"; impossible to guess).
E.g. suppose the reader sees 帰納. It is very likely they can read it correctly as kinō. The problem is not knowing what it means, that it refers to (mathematical) induction, or inductive reasoning. The dictionary lookup is trivial, though.
Basically, someone fluent in Japanese can read unfamiliar words like that most of the time. They will know a ton of words in which 帰 is ki and 納 is nō. In situations like that, it's not much different from someone encountering an unfamiliar word in English or Spanish text.