The problem with class-based RPG systems is very simple. There is artificial growth in abilities that take time, training, and use to improve.
Take for example the classic Fighter and Wizard characters. At 1st level they both have a proficiency bonus of +2. While a Fighter is proficient in all weapons a Wizard has a very limited amount.
At 1st level a Wizard might use a weapon more than spells. But as that Wizard advances in his class level he become more and more dependent on the arsenal of spells. The weapon at 1st level gets used less and less.
Fast forward to 18th level. The Wizard and the Fighter both have a proficiency bonus of +6. The Fighter by nature of his class trains and relies on weapons. His bonus makes sense.
The 18th level Wizard hasn’t used the weapon in many levels. His spells are his weapons which are far more potent than any mundane weapon. Yet he has the same bonus as the Fighter.
It’s just an aspect of class-based systems that bothers me. It’s not realistic at all. And there needs to be some realism to make a game believable and to be able to rationalize any unbelievable or fantastic aspects of it.
This is why I like skill-based systems over class-based ones. Although I have and do play both kinds of games.