Learn English – The opposite of ‘A coin has two sides’

An idiom that could be useful here is that of the double-edged sword, which indicates that something positive and useful in one scenario can be negative in another, and may even be used against you. A double-edged sword cuts both ways. The idiom is typically used in situations where the negatives are not obvious, and may come as a surprise.

Working with the example, you could say something like this: "For all of its advantages, ABC is something of a double-edged sword." Then proceed by describing disadvantages of ABC.

'Two sides of the same coin' does not quite mean what you describe.

two sides of the same coin - different but closely related features of one idea

It essentially means that two things are the same. I might use it in a context where someone is describing someone else as both 'lazy' and 'messy' and in response I could say that those are 'two sides of the same coin' and thereby suggesting that they are one and the same, in this case suggesting that the person is messy because they are lazy.

As far as I can tell you want a pithy phrase to describe two things that are good together but not necessarily either good or as good apart.

I'd suggest a simile in this situation. A very common type of phrase is to say:

[Something] without [something] is like [something else] without [something else].

Currently on the London Underground for example there is an advert that says something along the lines of 'a woman's hair without product x is like rock without roll' (the latter part usually a ridiculous separation for humour value). There's no standard phrase used here but it's an opportunity to be creative.

A couple more examples:

A man without ambition is like a bird without wings (from a 1908 business magazine)
A house without books is like a room without windows (a proverb that goes back even further)