Unfortunately this is one of the areas of the new payment system where I have more questions than answers, partly because eBay's system is new and untested, with policies that are different to other sites when it comes to disputes, and partly because eBay has not exactly made the info freely available and / or easy to understand, or extrapolate a decent estimation for, how it will work from the info they do provide.
The similar payment systems I currently deal with on other sites are a lot more clear about what is and isn't available in so far as protection goes, and whats required to qualify for that protection.
On Shop*fy, it's basically like any other merchant facility, no real protection, so I lose unless the bank decides against the buyer.
On Et*y, who are much more akin to eBay than the above site, they do have seller protection for both regular disputes and chargebacks, and cases can be opened within 100 days of the sale (if they are not escalated to Et*y for review, they can also remain open for a full year if no resolution is reached, so that's a thing... lol) but proof of post is required, not proof of delivery, and that's the kind of information that can be obtained and kept indefinitely. This site applies their seller protection policy to all disputes made through other payment providers, too, so individual payment system's policies are irrelevant - basically, the seller's financial contract is between themselves and Et*y, so those T&Cs always apply, and what happens between (for example) Et*y and PayPal doesn't matter, but I find this a little more acceptable in that scenario because the requirements for seller protection are either equal to or less than what is required by the original payment method the buyer would use, which in turn means Et*y are making us use their payment services, but in doing so they are providing genuine protection with requirements that are not above the requirements of other payment systems.
eBay has made no mention or indication they intend to follow suit, which is troubling - when I'm on managed payments, my financial contract and the T&Cs that apply will be with eBay, no matter what payment method the buyer uses, therefore I have to assume eBay's seller protection, and T&Cs of the MBG will be all that can apply when it comes to disputes / chargebacks.
I feel this should require some adjustment to those terms, particularly for disputes initiated after 60 days, but for now it's a bit of a nervous wait and see situation.
