If you are not paying for Sync you are getting ads. So you are still paying (attention, data, you are the product).
Additionally proprietary software has some long term disadvantages that are not visible at first. This is why one needs to always inform people about it as they do not understand these disadvantages.
The biggest issue is that the owners of the proprietary code can change its course whenever they like it. See for example the Reddit Apollo Dev who would have sold out the community for 10 million dollars:
See his own published audio and the discussion on HN: