I'm splitting hairs, but actually this is not true, while in practice it might seem so. There is no guaranteed delivery of SMS/MMS messages in the standard and if the message is not delivered, you might not get notification on this as most of the operators (at least in U.S.) do not support notification reports, which would tell when the message has been received. There is no guarantee that the SMS/MMS will be instantaneous, which is why you will see cases where some messages are lingering somewhere in the systems hours or even days before they are delivered.

So while it is true that there are far more SMS/MMS capable devices in the market than email and I think nowadays MMS is actually configured properly on most devices to work at least inside that particular operators network, there is no guarantee that it will ever be received by the recipient and absolutely no guarantee of any instantaneous delivery.