I also have issues sending video over MMS.
I think the issue is just the design of MMS. It was designed back when dumbphones ruled, so it was designed to have cell networks transcode media into a compatible format for the target device. The cell networks don't want to spend the cpu cycles or perhaps engineer time to do it properly. Perhaps RCS[0] solves this issue, but I think everyone will have switched to WhatsApp, et. al by then.
[0]: https://en.wikipedia.org/wiki/Rich_Communication_Services
I just used it on my S7 and shared it on whatsapp: looks very high quality to me. You can try changing the gif quality from settings, although the default worked fine for me.
I tried at the medium (default) and highest. Oddly enough the highest quality setting was the most blocky so I'm guessing the MMS or Pixel->Iphone text did something weird?
Either way, the app is neat and I think this is just an artifact in the MMS stage rather than the files.
Is this an MMS issue or something?