Likely this will filed automatically under "unanswered" but here goes…
A well documented flaw in Squarespace (still) is how video banners don't play on mobile.
Squarespace has a fairly misleading answer to this on their Support site:
The reality is that banner videos won't, under any circumstances, play on a mobile device (that I've seen)
This a huge drawback to the platform that at least two other CMS (Content Management Systems) have seemingly solved — Cargo and WordPress.
Examples of these two working on mobile are here:
Obviously Squarespace provides a host of other wonderful features but as video becomes (I would say is) the common visual language it seems like something they should be able to devote some time and energy to fixing.
Fallback images are a poor substitute, especially if say, you are a video production company. GIFs as Fallback images while clever are large files and won't play nearly as smoothly.
Finally, what's really infuriating is they've gone to the trouble of solving this for their own marketing on their their 5to9 campaign site, the irony being they're selling the service with a feature you can't actually use on it.