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".