As long as your clients are equipped with phones able to display SMS or are reachable (meaning in an area where they are connected to a carrier mobile network), you should be able to reach them on many existing phones.
Please note that if your client opted out to receive your messages, you might not be able to reach them.
Most people are equipped with smartphones nowadays so the reaching base grows bigger every year. But not all do, some old phones and people who do not have MMS enabled in their cellular plan are unable to receive them.
What about the carriers? Do they have limits?
All of the carriers provide delivery reports, at least for shortcodes. Each carrier also has their own limits, but files under 1MB are generally safe for all major carriers in the US.
Here is a non-exhaustive list of carriers supporting MMS with shortcodes for the USA:
Metro by T-Mobile
If you want to have more information on supported carriers, please contact your Account Manager to get the latest updates and raise your need.