I tested on an iPhone 5, Nexus S and an iPad 2. Most things worked well across the board but there is some weirdness. Eg "what's 2 + 2" returned the correct result on the iPhone 5, worked on the iPad 2 after several tries and just failed every time on the Nexus S.
Some times the failed was result was just weird, but most often it was "what's to plus 2".
Did you use it with a headset or via device microphone?
The noise-correction on the older device microphones aren't as good as the newer ones (ie, anything that's Siri compatible has at least 2 microphones for noise-reduction).
So perhaps Apple does have a legitimate (although self-serving) reason not to deploy Siri on older tech - it may suck if you have lots of ambient noise or distortion.
I also have a very poor experience with my Nexus S when using Google Now and voice search. Unfortunately, I don't believe that the Nexus S was speced to be able to run 4.x. I get the impression 4.x was designed for newer devices.
Some times the failed was result was just weird, but most often it was "what's to plus 2".
What's going on? Still more impressive than Siri.