Cantonese and Mandarin are mutually unintelligible. China likes to call them different dialects for political reasons, but for all practical purposes they are different languages.
If you're talking about spoken, probably the difference is something more like English v German.
Individual words can look similar but combine them into a sentence and you probably wouldn't be able to parse it on a first take (if they were transcribed into romanized letters), and there are some grammar differences.
It's even worse than English v German imo (speaking both, and descendant from Cantonese, married to a mainlander)
The german grammar is a fair point though. Makes every sentence sound like churchill trying to be annoying on purpose.
It's like English v German, but imagine that German had a bunch of sounds that seem identical to you but are actually different sounds (you just can't hear the difference). Imagine that German Wasser (water) and Wäscher (washer, ish) sounded exactly the same to you, and you couldn't tell the difference. Now imagine that literally every single word had dozens of those identically-sounding-but-actually-different-sounding-to-native-speakers examples.
The issue imo is the increase in tones. Mandarin has 4-5 tones, cantonese has like 6-9. And the tones in Mandarin are much easier to distinguish.