Sync is only to prevent access points (or masters in a PtP) that use the same, overlapping or too near frequency from interfering with each other. So any backhaul masters or APs that can see each other and are using the same or over lapping or too near a channel need to be synced so they become invisible to each other. Normally you only use sync when you are reusing frequencies on AP’s / PTP Masters that can see other and it’s assumed their client radios are not able to see the other AP’s/Masters (except maybe at power levels low enough they don’t interfere).
It does not stop the clients of all these AP/PTP Masters from trashing anything that can see them. If you have two APs/Masters synced but the client from one of them can be seen by another AP/Master running in the same, overlapping, near frequency at high enough RSSI then that client can interfere with that other AP/Master and that AP/Master can interfere with that client.
If you don’t need sync the first advantage of not using sync is that sync locks you to, at most, 75% down (and 25 up). So, (and I’m using 100mbps just to make the math easy) if you have a link that can do 100Mbps total and you split it 75/25 that means your link can never do more than 75mbps down(tx) even if there are only a couple of bits being uploaded because that other 25mbps is reserved for uploading (rx) whether it is being used at the moment or not. If you did 50/50 then you would never be able to do more than 50mbps down regardless of the amount of up being used at any given moment.
If you are using ePTP (or TDD flexible if that’s still a thing) mode there is no ratio, nothing is reserved. The radio can use nearly all 100mbps for upload or download, whatever the moment demands.
Also I believe ePTP should have noticeably less latency since doesn’t have to wait on specific timing to tx or rx.
We only have 1 550 link and it’s not bonded. I don’t know what the rules are on that but I would assume sync doesn’t help here since both client radios at the other end are going to see both Masters and vice versa so I’m guessing different , non-overlapping with possibly larger than normal guard channels are required here ? I’m just guessing though I have no idea what the rules for bonding 550s are.