Im guessing "unlimited" sms, is also similar to "unlimited internet", you start sending more than a 1000 sms messages a day you might get throttled?
Were I work, we've implemented a couple of nation-wide general population surveys regarding non-communicable diseases (working with those countries health authorities, of course). For the actual surveys, we get SMPP contracts with the local telcos because of this limit (and to get a vanity shortcode, reverse billing...).
We started thinking it would also be cheaper, too (due to the scale pricing) - but the human time required for working out contracts with developing-world telcos for a project involving state actors can easily counter that.
The throttling is something we need to take into account for the tiny-scale pre-tests we ran (while working out the contract) using an SMS Gateway App[0] we've built, similar to the one from this thread. The main difference is ours uses a queue-based protocol to exchange messages with a controlling server.
A "person" can't reasonably compose and send more than 3 or 4 messages per minute on average. Send "too many" as defined by the carrier and you will likely be cut off.
I received the bill at the end of the month stating the timestamp and recipient of every message. The PDF was regularly around 100 pages long.
This is usually (at least marketed as) a spam prevention system, so you don't blast people with a really cheap, disposable SIM card.