I’ve only used Alexa in stand-alone units, and Siri on my phone and watch. For me, Alexa regularly fails, and the overwhelming majority of my interaction attempts play out like this:
Me: Alexa, Badezimmer 10%
Alexa: Ich kann nicht Prozent auf Spotify finden
*Five attempts later, it actually works*
(It was even less reliable in English)
Siri mostly hears the right words when I ask questions, and the voice keyboard (is that the same speech-to-text engine?) is correct somewhat more often than the swipe keyboard (although that doesn’t say very much as the latter seems to have become very confused as a result of me learning German).