One-way versus two-way
One-way sync means you push rates and availability out to the channels. Two-way means the channel pushes reservations back into your system, which then deducts inventory and tells every other channel. Without that return path, each intermediary booking still has to be typed in by hand — and that is the gap where double bookings are born.
Three things need syncing, not one
Availability is what everyone thinks of. But there is also rate by room type and by date, and selling conditions such as minimum stay, cancellation deadline, and opening or closing dates. If only inventory syncs while rates are edited by hand on each channel, every price change becomes an evening of retyping, and missing one channel means selling your peak season cheap.
Latency is the number that matters
The right question is not "do you connect" but "from the moment a guest books on an intermediary, how long until that room is deducted everywhere else". A few seconds is safe. A few minutes is enough for two guests to book the same last room on a Saturday night. Ask for that figure directly, and ask what happens when the connection drops.
What happens when the connection drops
Networks fail, channels go into maintenance, and providers change protocols. It will happen. What you need to know is whether the system queues and resends once the link is back, whether it tells the person on duty that sync is broken, or whether it stays quiet and lets you discover it while a guest stands at the desk. A good system is loud when something is wrong.
Commission and the direct-booking question
Intermediaries take a commission on each booking, so every room sold through them has a noticeably lower collected rate. That is not an argument for dropping them — they bring guests who would never have found you. The sensible approach is to let intermediaries create the first stay, then keep the guest in your records so you can invite them to book directly next time, where no commission applies.