English is Germanic.
Therefore we have unmodified adjectives functioning as adverbs.
This usage doesn’t have unlimited application though.
Generally the best adjectives are short (in terms of syllable count) and primitive (rather than borrowed or derived).
The consequence is that some adjectives sound wrong when used in certain roles.
It might be that the mere availability of a possible alternative (“hard”) makes interpretation of the explicit adverb more difficult than otherwise. If so “hardly” is wrong because there are two opposing meanings to the adverb and one other meaning to the adjective—and the alternatives don’t have the same explanatory functions in clarifying the meaning of the sentence.
Clearly two non-syntactically equivalent items cannot be used in tandem with a single conjunction (for instance).
The attempt to parse your “hardly” statement may just end without any clear candidate representing the intended meaning of the sentence.
The computer works—hardly. (Minimally functional.)
*The computer works electronic. (Long, non-primitive adjective fails as adverb.)
I work hard. I run hard. (Intensity. Short, primitive adjective succeeds as adverb.)
I work—hardly. (Limitation of degree.)
I hardly work. (Limitation of degree.)
*I work hardly.
1. Check superficial plain meaning: limitation of degree? Possible.
2. Contrast to alternative meanings/constructs:
Is operational? Reduced applicability to personal agent/subject.
Exerts itself? Possible.
In a hard manner? (I.e., as the adverb denoting the manner corresponding to the adjective.)
Resolution of meaning
No direct comparison possible. Failure.