One problem is you need separate infrastructure to host your monitor. You also need to monitor the monitoring service, or it is easy for it to be quietly failing until your real site fails without warning. We run a separate instance that only monitors the public instance of t1mr.
Attention to detail matters and you really want to be focusing on your product. We are quietly handling other stuff, like calling multiple people until someone really answers the phone, or checking if your ssl certificates are about to expire.