It's not so much about security, as FOSS conversation groups would be open to anyone anyway, but it's not a good look for a project to use a tool that is known to be quite shady while there are FOSS tools, or simply tools with a better reputation. Also the project group seems to be french, not english-first-language, and Telegram is absolutely not well seen in France, not used by much more than a few percent
If you say "shady" because CEO is a Russian in Dubai and for years nobody really knew how the hell could he sustain the company, yeah you're right.
About the FOSS alternatives you're right, but to use a closed source SaaS is just a choice people make because they are not confident in their own infra.
If it's "shady" because of cybercriminals, I insist in saying that it's the same than using Discord (pedophile rings) or Whatsapp/Meta messenger (extensive history of terrorists, gangs, traffickers of any kind).
It beats WA on UI in most cases (especially on desktop), has open source client, much better groups/channels for one-to-many, many-to-many communications. Has bots support like I never seen on WA.
