Some of the features might be redundant (at least for some use cases) so it's not like "missing features" is necessarily a deal-breaker.

Not that you said so, but it's something one can think when he hears that Riot might be "missing stuff".

They have a different approach. There are some details under "Same but different" part of comparison to React. https://muut.com/riotjs/compare.html

The design is vastly different. DOM update logic is more straightforward. We realized that heavy diffing is not needed.