My problem with Siri is that it doesn't understand speech. I am non native speaker but coming from Android I had no problems with Google in understanding what I am saying. It just does not understand or completely alters the phrase.
Unlike Google Assistant, Siri comes from the Apple ecosystem. Apple has a very weak market share in non US countries. Google is where it is because of its huge market share and their ability to capture a lot of data.
If Apple is adopted more and more, I dont think Siri will be so bad. They can also train it better with non native accents.
If you're not a native English speaker, have you tried changing the langauge in Settings > Siri & Search? Either to your native language, or to a regional variant of English that more closely matches your accent?
I am learning Spanish and the iOS translate app can’t understand my very bad Spanish. It understands my native language - English - almost perfectly.