It's had a similar number of issues, as compared to Maps. The difference is, it didn't replace or introduce a must-have feature in iOS. As a result, it's failures were something users are able to overlook because Siri is still, essentially, a toy feature. It's not key to the phone's operation; heck it's non-obvious that it's even a feature because no where on the interface indicates that it exists (it's a hidden long-press action on the home button).

Maps, however, is crucial to most people that use an iPhone. After telephony and web functions, it's probably the next most used feature. So, any failure by it, even if it's slight, is amplified. Yes, you can use Google Maps on the web or another app (MapQuest should really be capitalizing on this opportunity to reclaim some relevance), but it's one of the default icons on the home screen and where most people are going to head first. It would be like the dialer not being able to call certain phone numbers or the text message not working with certain recipients. It's a vital function and has to work.

I would say Siri is probably more of a failure than Maps, but because of it's non-essential nature, that feature is able to get away with what Maps can't.