The point here is that you're using generalities that are wayyyy too broad.

There's no reason a body on frame vehicle can't be stout. See every unibody fullsize van for example.

The fact that you think the Chevy Avalanche[1] has unibody problems says everything I need to know about your opinion

There's nothing fundamentally wrong with unibody construction for a vehicle like this some work vans have been that way basically forever (though it makes me want to go postal when the OEMs graft a really shitty frame onto the back of a unibody van and call it a "cab and chassis").

[1]https://www.gmwholesaledirect.com/v-2009-chevrolet-avalanche...