Just set mine also up with a Sonos 5. It seems like you can play music through echo real loud and the echo can hear very well. Thus, sound cancelling seems to work.
However, like your experience, Sonos cuts out the Echo response a fraction of a second or more. This is very annoying.
I also don’t think the fidelity is as good as Sonos by itself. This could be because the Echo is a “Pre-amp” output to the Sonos. If I recall, a pre-amp should be set at 3/4 power and then only the amp should be manipulated. Using the music through Echo requires keeping Sonos (amp) value constant and Echo (pre-amp) variable. For this reason, I tend to think the fidelity issue will be an issue with any speaker hooked up to Dot though maybe more or less noticeable than Play 5.
For me, the negatives out weigh the positives. I like the Dot, but not with a Play 5. Ideally, Sonos will integrate Echo into its architecture so it can use the noise cancellation and voice control natively.