Dynamic timeline cards - any downsides?

With the recent addition of the "Enable dynamic cards loading", they started pre-loading with a placeholder on initial page load, and it looks great! Also appears to load slightly faster. 

I have 2 questions about it:

1) Are there any downsides to using this? What would be a reason one would choose to not use dynamic cards?
2) Does this work for loading new pages/switching to other feeds? Right now it appears that it works only for the initial page load, rather than for adding new items via load more or switching feeds.

About the https://github.com/unaio/una/issues/3026 brief content cards:

Visually not much changes, except embeds of vimeo get cut off, so no-one can use the player, what exactly does this option do?