This system is good, but one issue/concern with this is the cost.
I would like to know which of "issue" and "concern" is better for the above sentence I created. That is, I am searching for a word that can be used in a situation where using "problem" is too serious.
Issue and concern, in the senses used here, are different, but can be used similarly. A concern is something to be concerned about, something that causes doubt or worry or similar. An issue is a topic for debate, or a problem that one might hope to solve. However, people don't usually think too much about the precise meaning of the words, they just use them in the patterns they are familiar with.
Either of them is acceptable here, and both fit the purpose you describe.
Concern and issue have very close meanings in this context and it wouldn't really matter. If you are looking for a more positive word, you could use "challenge" or "opportunity".
