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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.