I have not played DnD 5E so far, but I think PoE is much easier to understand than DnD.
PoE:
Hit chance = acuracy + 1d100 - defense (deflection, fortitude, reflex or will), the result tells if its a miss/graze/hit/crit so every point acc or defense increases your chances by 1% unless the difference is so extreme that an attack always misses or always crits.
Damage = base value * ( 1 + sum of all modifiers). PoE2 made it more complicated by adding double inversion, penetration and power level.
PoE2: passive effects stack, active and modal effects do not stack
DnD:
- What stat is the attack based on (e.g. str for melee, dex for ranged, int for wizards, wis for clerics, cha for bards, there are lots of exceptions)
- How is the DC determined (which base stat, caster level, spell level, profiency, type of effect, sorry I am confused which DnD edition uses what rule)
- type of bonus ( not sure about 5E, but in 3E only bonusses from different types stacked (with exceptions) and some defenses worked only against some types of attacks (e.g. armor not helping against touch attacks))
So I would say:
PoE1+2 had a few rules that are relatively easy to understand, at least the basics.
Things get more complicated if you want to play on the higherst difficulty, maybe even for solo or the ultimate
DnD has lots of rules and for every rule there are several exceptions.
Most basic rules are relatively easy to understand, but to make a good char you have to understand most exceptions.

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