Actually it is very simple: adding Twitter to the mix adds one more "moving part". Shooting a simple email and then multiplexing it (via aliasing, for instance) to X recipients is easier and more reliable. Create an address like [email protected] and forward to X other destinations from there (including Twitter if you want), and send your notifications to that address using standard 20+ year old protocols and tools, instead of some 3rd party proprietary stuff which was "released often, released early" and hasn't crashed "in months"

It's slightly more nuanced than you're making it sound. For one thing, Twitter may very well be more reliable than your email system (YMMV). For another, email doesn't generally beep loudly and alert you.

The idea that an additional part automatically weakens a system is reductive. It doesn't necessarily weaken it at all.

Serious downtime and downtime are not the same, and if twitter is down for a few minutes that means a few minutes of not receiving an SMS because you're routing your services through them.

I've seen the fail whale 2 times today. Refreshing worked, but the problems there aren't entirely solved.