The mail is quite reliable. However, checks just seem to move through the system more slowly sometimes. Sometimes they even get lost.
A lost check can be dangerous. Your account information (those goofy numbers on the bottom of each check) can end up in the wrong hands.
Checks can be lost or compromised in transit, at a retailer, or at your bank. The more checks you write, the greater the odds of a problem.
Paying bills electronically and using a debit/credit card will reduce the chances of a problem. You can also get more liberal consumer protection by using a card.
Finally, you have to manually balance your checkbook yourself when you use paper checks. If you pay electronically, the bank does most of that work for you.
Further reading: