Warm and welcoming sure but too casual, you guys IMO implies familiarity, I wouldn't use it with strangers, could be regional or generational but that's my rural Illinoian take. Y'all is more flexible.
I agree with the other poster. Missourian and "y'all" is ridiculously less formal than "you guys". To me, "y'all" is specifically informal and is used in exactly that manner, even in corporate emails. It denotes a more conversational tone that's open to feedback. "You guys" does not exist within formal/informal for me, it's either, neither, or both, just depending on what you say around it.
"How are you guys doing today" spoken at a red lobster is absolutely fine, completely normal language, whether spoken by the president or by a child. It's the single most ubiquitous and wholly normal greeting that i know. Corporate really over does it sometimes
Fascinating - I'm from Michigan, and I would say "y'all" sounds more casual to my ears than "you guys." Formal speaking (in contrast to casual speaking) often eschews contractions.
