HTTP/2 (and HTTP/3) is vastly different in features (added multiplexing, windowing, HPACK etc). All this transforms a largely stateless connection (in the HTTP/1.1 case) to a stateful one. And in order to maintain the stateful connection, you need to store some data (state, configuration etc), thus all these problems.

Also, in HTTP/2, since multiplexing is added, the protection characteristics are different. For example, if the connections were generated by CDN source draws, you may just allow fewer number of connections each with a large pool of multiplex channels, but if the connections were from direct user access, you may then want to allow large number of connections, but each with fewer number of multiplex channels. In HTTP/1, protection is much simpler, since everybody looked almost the same.

HTTP/1 has lots of unobvious edge cases, and legacy quirks. Text format has way more flexibility than it seems from valid headers. It has obscure features like multi-line headers and old MIME features, 100-continue race conditions, custom hop-by-hop headers, GET bodies.

Fortunately new HTTP RFCs document many pitfalls. If you just implement what RFC 2616 said, you won't have a safe implementation.

Actual size of a request or response can be specified in multiple ways (at the same time, with conflicting values), and depends on a combination of several features, and values of headers with weird parsing rules needed for backwards compat, so "simple" HTTP implementations can be tricked into request smuggling.

Either way you need a robust, well-tested mature implementation.

For normal asset serving the only advantage is more assets can be loaded in parallel since HTTP/1 is limited on connections per domain. CDNs on different domains usually prevent this from being an issue.

In theory you could serve unbundled JS assets via HTTP/2, but I have never seen it in production. Likely because you still need a compilation step most of the time.