The inherent problems I see stem from how many things begin to "add up" above 8th level or so. Attack bonuses scale poorly with AC bonuses, Saves scale poorly with Save DCs, Damage scales poorly with hps, etc.
For example, at about 6th level, the party's main front-line warrior is likely to hit the Bad Guys ~80% of the time, and that hit is likely to moderately damage the Bad Guy.
By 12th level, the party's main front-line warrior is likely to hit the Bad Guys 99% of the time, and that hit may very well drop said Bad Guy.
And by 20th level? If the player knows what he's doing, 300 hp of damage per round is easily within reach. Put that together with the rest of the party, and a 800 hp dragon is dead in round one....before it acts.
(...and of course a "good DM" can handle it. I do so all the time. That's not the issue; the issue is "should the DM have to?".)