The idiom is a set of words established by usage as having a meaning nondeducible from those of the individual words (e.g. Over the Moon, there is a light (bearing on its surface the darkness behind the moon is not visible on Earth). What is the light? “To do more harm than good” has an obvious meaning deduced from the individual words, thus I would not call it an idiom. Typically the expression is found in formal writing, especially of a medical type, including when the balance of benefits and drawbacks of medication or other treatments are being discussed. Thus: (i.e. the absence of any specific medical treatment should remain unresolved through a qualified person) Is immunogenic nutrition good for more than just sepsis patients at this time of the year? Idiom

(Cambridge Dictionary) –

” / / 1 / 1 / 1 / 1 / 1 1/ 1 / 3 / 1 / 2 / Idiom, 1 / 1 / 1 / 4″ ” Idiom’: ” (Cambridge Dictionary.)

Using “massed” in place of “crowded” as in your example sentence is not a correct usage. Also some use of “massed” has been suggested (along with other usages) I would just like to

use the word “packed”.

“the skills I acquired are knowledge in A, knowledge in B, knowledge in C.” The second and third occurrences of “knowledge in” may be omitted, but the things acquired are still multiple.