Group texting works fine for me. It auto converts to MMS and sending/replying works as expected. On my old Kyocera, a group text would seemingly send individual texts in a loop over the recipients, which always seemed broken.
The one maddening thing about texting on my Sonim is the KT9 predictive texting. Short, high-frequency function words such as "am", "of", "the", etc always have way decreased preference compared to larger, less common words that start with those substrings. e.g. If I type "amethyst" just one time, that word will always be preferred over the much more common "am".
This experience matches mine. Never had problems group texting. Also find the predictive text a little frustrating (it's also seemingly impossible to reliably type contractions like "I'm"), but not deal-breaking. I see in another comment here that there's an open source T9 Android keyboard [1] that I may try out...
The one maddening thing about texting on my Sonim is the KT9 predictive texting. Short, high-frequency function words such as "am", "of", "the", etc always have way decreased preference compared to larger, less common words that start with those substrings. e.g. If I type "amethyst" just one time, that word will always be preferred over the much more common "am".