D20 system at face value is fine. It simply is a way to quantify a randomness and relative potency of modifiers.
The issue that can cause aggrivation tends to be the "Natural criticals" aspect of D&D. Where, irregardless of DC and modifiers and Nat 1 is always guaranteed to be a miss/fail and a Nat 20 is always guaranteed to be a crit/success. Which means that no matter what you do, you're always subject to a 5% chance to get screwed.
Which can be frustrating. Especially in extreme cases like you're rolling on a 5 DC skill check and have +16-20 modifier that should absolutely definitely guarantee you are way more capable than necessary and you still fail.
Worth mentioning that in current 5e, natural crits and fails only apply specifically to attack rolls - critical lucky/unlucky moments in the heat of combat (and if your attacker has a -10 on their core ability when rolling the attack, then they are not going to do much damage to you anyway, even if they do crit and get a lucky shot in). They do not apply to Ability Checks or Saving Throws. Larian are the ones who decided to ignore this and apply crit success and failure to all dice rolls, so you can blame them for that, not the system.
In a proper implementation of the system, the character with a +14 to their ability check can never and will never, ever, fail a DC 15 check against that skill... and the demilich with a DC23 spell save can never and will never fail to dominate the crowd of peasants with no wisdom bonus.
