The compatibility hazard is negligible; hanging punctuation is the main one that can actually cause harm if misapplied, and conservative heuristics for enabling it are easily arrived at.
You might be surprised at how little of browsers’ existing functionality in these areas is actually defined, or in progressive cases how much is defined as being up to the browser.
When I mention things like Knuth-Plass: all current browsers use a greedy first-fit line-breaking algorithm, but IE actually had something better long ago in some situation (I forget), and until recently, the entire thing was simply undefined. Now, the default is declared to be UA-defined (text-wrap-style: auto) <https://www.w3.org/TR/css-text-4/#text-wrap-style> with the option to declare a preference for alternative modes like stable, balanced or pretty. Long before this, Firefox had a bug open about implementing Knuth-Plass or similar <https://bugzilla.mozilla.org/show_bug.cgi?id=630181>, and the only oppositions have been on technical implementation grounds, not that there’s anything wrong with the concept, which people generally agree is desirable.
As regards heuristics, you can define the heuristics to use completely, or you can leave it to browsers with some suggestions, or you can leave it to browsers. There’s plenty of precedent for all three of these.
> hanging punctuation is the main one that can actually cause harm if misapplied
Yeah, it can screw with layout. We recently removed it because it had seemed harmless to have it enabled so Safari users could enjoy it and other browsers would just ignore it, but then we discovered it was screwing up the layout on the main page's lists (https://gwern.net/index) and was not harmless after all. :(
