Note - I may take a few days to recieve your email. Hotmail filters all new emails as junk. Would be best to post all questions, concerns in a forum topic to catch my immediate attention. This way others can assist and also correct any similar mistakes.
There's no hard and fast rule, some MOD's are well written and coded using good standards and techniques, others, are quite simply terrible and if they don't cause issues initially, they probably will eventually, things like objects and connections not destroyed etc or never tested with different database types etc etc. Many MOD's are not maintained either so may run into issues when things like new versions of the database are released, MySQL is a good example of that becuase of the considerable changes even between 4.0 versions and 4.1 versions of the database engine some MOD's no longer work out of the box without tweaking from what I've seen/heard.