One of the quality-of-life things I really miss about MacOS when using KDE is the dictionary quality, and in particular the ability of Apple to include random technical words (aplanatic?), recognise foreign words being used out of context (I like Dansk øllen) and silently correct things like German obscure capitalisation rules.
Unfortunately, aspell / hunspell just aren't in the same ballpark – and not just because of the lack of words. I think the dictionaries are much smaller and it's harder to set up this kind of weird, but very useful, behaviour.
I also really like how easily available it is; in any app (except games), just put the cursor over any word and press harder than normal on the trackpad and a bubble with good definitions appears over the word. I don't think any system comes close in terms of dictionary ergonomics, for whatever that's worth.
There is also a difference between a dictionary used for spell checking and a dictionary for definitions: smaller (within reason) is better for the former, while larger is better for the latter.
(It is better to add an uncommon word to the user dictionary than it is to have a large number of uncommon words in the system dictionary since it is better to alert the user to a potentially misused word than to leave a misspelled word in place. Of course, this doesn't help in every case - as is evident with the common auto-correct snafus.)
Unfortunately, aspell / hunspell just aren't in the same ballpark – and not just because of the lack of words. I think the dictionaries are much smaller and it's harder to set up this kind of weird, but very useful, behaviour.