As noted in the manual, Sync object does not send data all the time, and you can also adjust the frequency of messaging yourself too with the bandwidth setting. In the lowest bandwidth mode, or if none of the data is changing, it will only send a message every 2 seconds, which is a tiny amount of bandwidth but still enough to bring any new joining peers up-to-date quickly. So it already automatically optimizes to only send on demand.
I'm not aware that adjusting the frequency of messages has any bearing on cheating. Usually the authoritative host model is the best mitigation for that, which is the architecture the Multiplayer feature uses.