Myth check
If I memorize radicals, I can guess every character.
Radicals are a filing system and a hint, not a decoder ring for every new hanzi.
Verdict
mostly_false
Why
Many characters keep a meaning hint in one component and a sound hint in another. The “radical” in a dictionary is often just the lookup key.
Beginners get the most from radicals that actually repeat in words they already say: 口, 氵, 亻, 木.
Guessing wildly from radicals produces confident wrong readings. Confirm sound with pinyin you already trust.
Better framing
Use radicals to group characters you know, then meet new ones inside words, not as abstract puzzle pieces.
Practice tip
Open Radical studio, pick 口 or 氵, and say the example words aloud before you add a new character.