Honestly, it’s a couple things. First of all is just general compatibility. There are certain domains that don’t play at all (redgifs, for example). They just show up as links to click. Then, there are ones that are a little more transient, but still negatively affect the UX. An example is if you browse !videos@lemmy.world right now. The first post there (https://aussie.zone/post/6569161) won’t load for me in the feed, and will only load if I click into the post. Once it’s loaded, I can go back out and it’ll play in the feed - but no matter how many times I try or how long I wait, it won’t play in the original feed. Additionally, the play/pause, sound controls, and scrubbing controls are only available if I click into the post. They work, but it’s less convenient.

Here are examples of my experience in both apps:

Avelon (and I forgot to show you can scrub the gif directly in the feed)

Voyager (the video continues until after I click into the post - I just wanted to show let it load for a while)