matskralc
Explorer
There's also the fact that monsters vary in power. There are occasionally low CR creatures that are far more dangerous than their CR represents like wolves with Pack Tactics and trip or magma mephitis with heat metal or giants with high strength leading to better to hit rolls and damage. Not all creatures of the same CR are equally potent or powerful, which also skews building encounters based on CR.
This is extremely true, and it's why I don't blindly adhere to the math. There are certain creatures that I know my party can handle better than others. It's not really possible to take the handbook of general guidelines and add specific "adjust CR upwards by one if everybody is wearing metal armor and the monster can cast heat metal", though.
They do tell you to adjust difficulty up a step if the party faces drawbacks that the monster(s) don't, but I don't really know what more they can say about that. An extended discussion of examples could be more helpful, but I don't think they can really say anything that those of us who have been playing for a long time don't already know. Perhaps we could move the "monsters sorted by CR" charts to, I don't know, some place like the Monster Manual and use those pages in the DMG for that discussion. Add something stressing the importance of the action economy, too.