This is actually a broader question about word formation, how to build up words.
The site Bona lingvo has lists of same meaning words and among them are antonyms. A couple of frequent arguments are often presented to defend those words.
There are people who think that a certain word does not quite catch some meaning they perceive a word has. Native languages ...
Are there antonyms that don't being with -mal?
Have Esperanto speakers ever created antonyms which stand on their own, without being derived morphologically from their counterpart?
is the main question of your post, so I'll answer that first:
Yes, people (including Zamenhof) have created or do use (some) adjectives that can be seen as exact or ...
Two arguments I have encountered for preferring "x" to "h":
Because it is not an existing letter in the Esperanto alphabet, automated conversion is possible, e.g. sx could be unambiguously converted to ŝ. By contrast, even if combinations like -sh- are rare, they could occur in compound words e.g. dishaki, so sh cannot be reliably ...