The only real disadvantage of a composite key in MySQL is that MySQL has some severe limitations on when it can use such a key.
In particular, given your composite key of (article_id, member_id, created_on), MySQL would only be able to take advantage of that key (e.g., when one or more of those fields appeared in your WHERE clause) if the conditions in your query included a test for:
article_id and member_id and create_on (in any order, but all three)
article_id and member_id (ditto)
Any queries that used conditions other than those (e.g., a WHERE clause that used only member_id, or one that used article_id and create_on) would *NOT* use the index, so you'd be reduced to a full table scan.
This is well documented in the MySQL docs and is a limitation that may not appear in other databases.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.