Honestly, the whole "legit seller" obsession is overrated. Half the time, the official BSPs are just resellers themselves with a slightly better support contract and a lot more marketing. I've seen just as many horror stories from people who went direct to Twilio or Vonage as from the random WhatsApp "specialists".
That said, I'm not saying go with some dodgy guy on LinkedIn. but the key isn't official vs unofficial - it's how much control you actually have over your templates, your fallback routing, and whether the provider gives you direct access to the API errors, not just a support ticket that takes 48 hours to get an answer.
If you really want to pick from the usual suspects, sure, 360dialog or Dexatel are decent for template approvals and international traffic. Infobip is fine if you like complexity. But MessageBird's been slipping lately on support speed.
What nobody talks about: most of these platforms will screw you on pricing once you're locked in. The real trick is to negotiate a flat rate upfront and demand a multi-channel fallback (SMS, email) built into the contract - otherwise you're stuck when WhatsApp goes down, which it does, a lot.
So stop looking for "legit". Start looking for transparent billing and a support team that actually reads the error logs.