Let's just grant that 4E marketing was unconsidered(which I've already said) and 5e data was down to a T(well, not really, but grant it for the moment). Here is an example of a problem because solely how of how the game is marketed compared to what the game actually says:
Claimed Magic Items were not calculated into the math via marketing.
According to page 133 of DMG, a PC ought to get about 1 good permanent magic item per 4 levels and a consumable every level when you
work out the math. And specifically, someone interested in say hitting with a melee weapon likely ends up with about a +3 to hit/damage from a magic weapon.
There is no mention of magic items failing to change the math. In fact, when creating monsters from scratch, you lower the value of immunity to non-magical weapons based on level as an example. And for the most part, high CR creatures have a default AC of 19. That's fuzzy of course because of how 5e works, but that's the average AC.
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Now take a straightforward sword & board Fighter who does 1d8+5+2 damage with his longsword because he has a 20 Str. Take their friend who is exactly the same except she has a +3 longsword. They both fight an AC 19 Creature.
Fighter 1 has a +11 to hit(+5 stat, +6 prof), so he hits the creature on an 8(65% of the time) doing 1d8+5+2 or 11.5 = .65*11.5*4 = 29.9 damage is expected after 4 swings assuming no critical hits.
Fighter 2 has a +14 to hit(+5 stat, +6 prof, +3 magic), so she hits the creature on a 5(80%) doing 1d8+5+2+3 or 14.5 damage = .8*14.5*4 = 46.4 damage is expected after 4 swings assuming no critical hits.
Here's the problem:
It will take a group of 4 of the first kind of Fighter all doing Action Surge in the 1st round approximately 2.25 rounds to kill a CR 20 creature assuming the CR 20 creature doesn't take any of them out. i.e. odds are against them dropping the CR creature in round 2. Especially given two rounds of the CR 20 creature getting a chance to drop one of them.
It will take a group of 4 of the 2nd kind of Fighter all doing Action surge in the 1st round approximately 1.05 rounds to kill a CR 20 creature assuming the CR 20 creature doesn't take any of them out. i.e. odds are just barely against them dropping the CR creature in round 1 - and critical hits ought to make the difference on average. The CR 20 creature, on a poor initiative roll, might not even get to a chance to go.
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See what's going on here? One group of Fighters likely takes out an at-level challenge in a round, the other group of Fighters likely takes three rounds to do it. Magic items radically change how difficult a typical combat is going to be simply because a PC with one good magic item(and remember, that 20th level PC likely has
five, one of which is likely
better than a +3 weapon) doesn't behave in the same way that a PC without does.
Which is fine and not problematic at all provided the DMG tells you which party is at the correct power level in case someone didn't see the marketing about no magic items in the math.
Which it does not do...