A balanced encounter-building system simply means you know with good confidence (not perfect! just good confidence!) how dangerous a particular encounter will generally be for a group of characters. Because D&D includes both randomness and diverse options, perfect balance is impossible, even before we get to the fact that perfect balance is both not generally feasible in the first place and not generally worthwhile even when it is feasible.
You would need far less randomness to have any sort of confidence in such a system.
If we replaced the current attack rolls and saves with a d4 and used a bonus that scaled from 1 to 25 and then based damage on your ability modifier with no roll, then you might have some confidence in the outcome. The current d20-based system randomness eliminates any sort of confidence you can get in the outcome for anything beyond a trivial or overwhelmingly deadly encounter.
Two particular fights come to mind - When my 3rd level Goblin Artificer was killed outright in a trivial encounter and when my 8th level party defeated Orcus at the end of an adventuring day and after defeating his minions when we did not even enter the fight "full up". The latter was actually supposed to be a 1-round fight where Orcus was sucked back into the Abyss as the finale which was the end of the campaign, but we were doing so good the DM decided to play it out and we won outright.