We didn't "pick" UDP. We operate a VoIP related service that interoperates with many different carriers via SIP. Almost all of those carriers ONLY use UDP. SIP UDP packets can often be fairly large. This is especially problematic because GCE uses a non standard MTU size (1460 bytes). This does not make our app, or every other SIP related app that is forced to use UDP, "Buggy".
Rough summary of what I had here: I'm an engineer on GCE (in particular I built our current virtio-net device and a small fraction of the other fiddly bits that sit behind that) -- some details in the bug jumped out at me and I thought there might be a quick fix, but I hadn't processed all of the details and posted a bit prematurely. After further review my original post was essentially content free other than 'IP fragmentation works correctly between internal IPs', which is not germane to the actual customer-reported issue.
When I jumped on HN on my ride home I saw this and was a weeee bit over-eager to be helpful.