Using hints or knowledge at the app level is a much better experience if the app can tell the FS that two files are identical. The FS doesn't have to worry about hashing blocks within the file, correcting alignments, etc.
Hardlinks have long been a well-documented feature of many file systems. I don't see how their use in a context users largely do not meddle with is too confusing or complicated.
Anyways it can be both? It could let the filesystem handle it if it detected the FS has file or block dedupe and fallback to hardlinks otherwise. It doesn't but that could be done if it showed it was worth it.
Maybe it's because I've been recently playing around with deduplicating subtrees in a different domain, but I don't see what makes this feature too complex to be worth the cost. Sure, symlinks and hardlinks require some additional care compared to plain files, but it's easy enough to add tests for those cases.
...per unique TB written to datasets with deduplication enabled, of course, not the pool as a whole (even though that ram cost is indeed at the pool level).
there are plenty of useful things to be done with zfs dedupe at practically no cost if you design for it ahead of time. unfortunately for some reason people parachute into every thread where zfs comes up to say you can never ever turn it on, so people are taught not to even try.
for instance, i have a pool with plenty of datasets that have dedupe off and only a crucial few with it enabled: the ones that are nfs-mounted media storage volumes for fediverse instance containers that would otherwise all duplicate all media they see in common, which is most media posted to any of them and a good bit of the remote media received by any of them. the size of the dedupe tables in memory is purely a function of the number of blocks written to those few datasets and i expect to either sunset this laptop from 2012 or migrate the pool to a fresh one long before i would be worried about the memory cost of deduplication swelling a few orders of magnitude to where i'd have to care.