That reminds me of trying to teach my international student friends the pronunciation of 'uien', 'euro', 'eieren' and other arcane Dutch vowel combinations. The Spanish speakers picked it up with relative ease (meaning I only had to slowly break down the sound a handful of times), it took a lot of effort for English speakers and yes, it was basically impossible for German speakers. Saying "Onze Lieve Vrouwe Gasthuis" was considered a tongue-twister of epic proportion.
'De kat krabt de krullen van de trap' I think it's because German and Dutch are
almost 'fraternal twins' - they are so similar that the similarity can be misleading. I was always worried about accidentally using 'false friends' ('verstopt' means 'hidden' ('versteckt' in German), but is almost identical to the German 'verstopft' (clogged)), or to speak 'Germenglutch' (German vocabulary, English Grammar, Dutch pronunciation), rather than proper Dutch. For me it was even easier (or more confusing, respectively) since I'm from the western part of Northrhine-Palatine, so I'd heard in regional dialects some of the Dutch peculiarities that are defunct in modern standard German (like the 'continuous aspect').
Like the 'g' in 'gaan' superficially sounds like a hybrid between the German 'ch' and 'r', and 'ij' in Nijmegen is close to the German 'ei' etc.etc., so you sometimes have to 'unlearn' the German sounds to make place for the Dutch ones - and just when you start thinking
"Hey! This is hardly a foreign language, more like a dialect!", sounds like 'ui' or 'eie' sneak up from behind & try to strangle you.
German is more strongly inflected than Dutch, afaics (
https://de.wikipedia.org/wiki/Niederl%C3%A4ndische_Sprache#Grammatik), which
mostly makes Dutch grammar easier to learn for Germans than vice versa, IMO. What I found confusing at first: Dutch, like English, has a
continuous aspect that was replaced by a different construction in standard German:
'Ik ben mijn handen aan het wasse' (I'm washing my hands) vs.
"Ich wasche mir gerade die Hände" (
'I am washing my hands (right now)'). However, this continuous aspect appears in several German regional dialects, especially regional Rhenish dialects in my native Northrhine-Palatine -
"Ich ben/bin am Hände waschn" - and is sometimes used for comedic effect, or to mock people, and stuff like that sometimes felt weird at first, like 'Bad/Mock German'.
Weirdest word in the Dutch language: uitnodigen ('to invite (invite smb. in)') - to a German, it sounds like a combination of '(hin)aus' ('out (of)') and 'nötigen' ('to coerce'), so that one always gave me cognitive dissonance .... like 'You invite somebody in, so you can kick them out?'