I've heard the argument that you should never use SMS for a mission critical app just because of issues like this one.
That being said, some operators run a dedicated SMSC for M2M devices, so SMS traffic with their solution doesn't get "stuck behind" consumer traffic (like American Idol voting). AT&T does this through their partnership with Jasper Wireless, as does other Jasper partners. This is something you should ask your operator partner about.
(Disclaimer: I am a Founder and currently the CTO of Aeris Communications, Inc.)
Using SMS for mission critical M2M applications is certainly possible when provided by companies who understand the requirement and create dedicated networks for this purpose. For example, the company I work for (Aeris Communications) provides a high-performance SMS transport access to the CDMA cellular network for M2M applications, that do not have the kind of problem concerns you raise.
Thus, mission critical applications using SMS are not a problem for our Customers. Indeed, auto manufacturers are using the Aeris SMS transport for reporting crash notifications for vehicles, because their testing has easily demonstrated the level of reliability that they need for this purpose.
By the way, it isn't just the act of dedicating an SMSC for M2M - this does not avoid the inherent problems associated with the device roaming into other (perhaps smaller) networks. The intra-carrier SMS transport, as well as inter-carrier gateway providers that carry the SMS traffic can also get to be a problem, due to congestion from handset text message traffic, spam filters, queuing delays, etc.
Unfortunately, we can not guarantee that because it is ultimately the sim card providers' capacity. We do have devices which can control and provide feedback via SMS.