I assume it is bad style but I'm not sure whether it is grammatically incorrect to have an enumeration with different parts of speech (for example a prepositional phrase and an adjective) like:
"He was at a disadvantage and not good enough."
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The linguistic or grammatical principle that likely provokes your concern is termed coordination. Here's Wikipedia's article on the subject.
There often needs to be some kind of match between the elements that are coordinated in a sentence. But many of us poorly understand which kinds of matches and mismatches are grammatically felicitous, questionable, or generally unacceptable. In fact, it's a challenge for linguists to specify a parsimonious set of rules governing this matter.
Linguists often use the following as an example of elements which are mismatched in terms of sentence part categories, but permitted because they match in terms of function:
He's [a Republican] and [proud of it]!
A Republican is a noun phrase, while proud of it is an adjective phrase.
We might therefore wonder if setting them in coordination is permissible. It is, in this case, because although they do not match in terms of sentence part category, they do match in terms of function: They both complete (complement) the subject.
Your example does the same thing. The prepositional phrase and the adjective phrase both complement the subject, and they match in a way such that they can be acceptably set in coordination.
This doesn't mean that all complements can be cast as coordinates. As alluded to above, useful guidelines on this are hard to come by.
Whether or not your example is good or bad style is more subjective. If people who are good talkers were to debate it, surely they would want to know whether it was to be spoken or written, in which sort of genre, and targeted toward which kind of listener or reader. In and of itself, there is nothing bad about the style. Its meaning is clear. To me.
You have confused parts of speech with sentence constituents. A part of speech is a label attached to a lexeme or "word", like adjective, preposition, or noun. A sentence constituent is a label attached to a meaningful part of a sentence which can be characterized as playing a functional role in the sentence's structure, like subject, complement, or object.
There are situations in which elements in a set need to agree in some way, some situations in which it's better style if they do, and some situations which are generally seen as bad style, ungrammatical or just nonsensical if they are divergent.
He is red, of another planet, and father to 249.
The items in this "enumeration" or set of elements would be considered bad style by most:
1. Running the front office
2. Wrote computer programs
Ungrammatical or nonsensical (?):
The car contained peanuts, in, contain, and go tell your mother.
Their ages were 17, 32, 56, C, and yellow.
NOTE: Thanks to @snailboat for redirecting my original answer onto firmer linguistic ground insofar as identifying the relevant topic. She deserves credit for what I got right, and no fault for the answer's travels into Errorsville, Papland or Drivelvania.
"He was at a disadvantage and not good enough/insufficient."
First, I do not think you want to even use
insufficient. This word means
inadequate, not enough (Merriam-Webster), which isn't
not good enough. I would recommend using a different word or maybe a phrase like:
Second, I have never seen anyone use a
/ in formal writing to start a list or clarify a meaning. For just one word, you could use a comma:
He was at a disadvantage and not good enough, insufficient.
However, if this is more informal and grammar usage is more lax, you could use your
/. I would never use that in anything other than a personal email to a friend, as this punctuation seems out of place in that sentence. Also, it takes a moment to understand what you even mean by the slash, so it is probably better if you just use a comma (or you could use parentheses).
This is more of a question of style than anything. Thus, it might just be better to use normal writing convention and stay away from the slash.