The difference is, all that damage Luffy took on Thriller Bark didn't put him at risk of dying. Zoro taking in Luffy's pain, almost killed him. You're trying to compare what Zoro took on Thriller Bark to what Luffy took, but completely ignoring the affects it had on each individual. If the roles were reversed, and Luffy received that pain from what Zoro took on Thriller Bark on top of what he already received, you can't say with concrete certainty that Luffy would have almost died.
If Zoro was at 100 percent, and then took Luffy's pain, he would have been nowhere near the same condition that he was at by stacking Luffy's pain on top of his own. Zoro remained standing, yes, but was in no condition to do anything after that point, and if he didn't receive immediate medical attention, he would have died.
The same principle applies to the rooftop. Zoro took Haki, and fought through the pain to deliver one final attack against Kaido. After that, Zoro was dead on his feet, as the effects of Hakai caught up with him. If Law wasn't there, Kaido would have killed Zoro, and he would have been defenseless to do anything about it, cause he reached his limit of how much of that pain he could push through,