I don't have any experience with the Newton but I've examined the output of the previous 3 generations of iBike against PTs and SRMs. Every time iBike has released a new version they claim 1) that it is as good as an SRM or a PT, and 2) that it is an improvement over the previous release.

I've also spent some time examining the Stages compared to various other power meters. Like all power meters, it has good points and not-so-good points. Its data quality is not equivalent to a PT or SRM but it's not bad. I wouldn't use it for QA analyses, or for drag estimation, or for sprint training, but not many people do those kinds of things. Sky probably won't be looking at QA analysis or sprint training, and they can afford to send their riders to the wind tunnel for drag estimation.