Both Discourse and Rocket.Chat are superb. We make extensive use of them at the OERu (https://tech.oeru.org/intro)... I've written about why it's so important for open communities to be "pre-figurative" - to use means that are consistent with their ends (i.e. open all the way down) - rather than accept "expedience" at the cost of community principles: https://davelane.nz/arresting-slide-open-fauxpen
Of course, Heather, all the cautions you point out are valid, although it would be possible to get the impression that using proprietary software is lower risk than adopting open source software (whether using a weak FOSS license or a copyleft license)... you might want to note that all of the recommendations you offer are true (only more so) when incorporating any *proprietary* software as well. Basically, it's all just user-beware, as it should be, and FOSS is better to work with than proprietary software - at least with FOSS you're usually safe from patent infringement suits levelled at you by the FOSS developers.
Both Discourse and Rocket.Chat are superb. We make extensive use of them at the OERu (https://tech.oeru.org/intro)... I've written about why it's so important for open communities to be "pre-figurative" - to use means that are consistent with their ends (i.e. open all the way down) - rather than accept "expedience" at the cost of community principles: https://davelane.nz/arresting-slide-open-fauxpen