Very clever within its functionality
The disadvantage for me is that both sites currently need to have exactly the same addons installed and Mainio Sync prohibits updating a remote page if there is any discrepancy in addons across the entire site, not just those used for the content on the page being updated. I frequently have developer assistance packages installed on dev sites that I don't want to install on the respective live sites, so in those circumstances Mainio Sync unfortunately cant help me.
For that limitation, I would currently rate the functionality provided at 3-stars. The code is so well put together and the interface so slick that I have given it 4-stars.
Should a future update to Mainio Sync provide a way of synchronising pages where only addons relevant to the content being synchronised need to match, I would happily award 5-stars.
We are happy to look further to your request in case we hear this need back from the add-on users. Just to clarify why we made this as a requirement at this point (for others): this is because we wanted to avoid dealing with situations where the content would be un-synchronized e.g. because of a missing block on the page or the site is not functioning correctly because the originating instance had an add-on installed that affects the site globally without being connected to any blocks on the page (e.g. a script that globally adds some JS to the site). Some of these would be hard or even impossible to catch simply by doing a detection based on the blocks and attributes on the page. And if you allow some exceptions to the rule, it might lead to an endless route of problems with some configurations. This is why we encourage both instances to be in matched states when doing the synchronization, to ensure the best possible result.
We do understand your point and as mentioned, we are happy to revisit this point in case we hear more people disagreeing with our line of thought. We would still see this as possibly a more developer'ish feature, so in case we'd implement something like this, we'd possibly make it configurable so that the more inexperienced users could not mess up the synchronization too easily.