The only downside (in my view) to sending directly to Omega, as opposed to having an AD or OB do that for you, is that you have no safety-backup for return shipment. What I mean (and say this from personal experience) is that Omega will ship the watch back to you whenever service is complete. They’re not going to confirm the date with you. So if you’re out of town for a week when the watch gets shipped back (or can’t get off work that day) and can’t be home to sign for it or can’t make other arrangements (with a relative or friend), too bad—FedEx (or whoever) is going to be in possession of your $$$$ watch, doing lord knows what with it that could end up creating a headache for you. Having the AD or OB does indeed add a middleman and deprive you of direct login credentials to track the service—very true—but they also receive the return package and can keep your watch in their safe until you can get there to retrieve it at your convenience.
Not saying one option is better than the other. I’ve done both. But especially if you live a life where it’s hard to predict where you’ll be on any given weekday three months from now, direct sending might have its tradeoffs.