The difference between his last two reworks (I haven't been playing long enough to experience any reworks before that) and this current iteration is that old Ryze was simply a ball of stats and it wasn't always clear what to do as him or against him.
You would have tons of damage on your spell combo almost entirely on point-and-click abilities, be able to heal off of it due to the spell vamp on his ultimate, and you also had a mana shield to not die while sustaining up. His spells all had similar animations, leaving his opponents not entirely clear what spell Ryze was casting, while the combo he did wasn't always clear to players that hadn't mastered Ryze. As a result, Ryze was devastating in good hands but complete garbage for everyone else, and that made him impossible to balance.
The Ryze rework fixed these problems. All three of his basic abilities have distinct animations that makes Ryze's opponents know exactly which ability he is using and he has a large variety of combos that give rather specific output that newer Ryze players can understand. The variety of combos he has also turns his gameplay from optimizing a single combo to understanding which combo you need and when to execute on that combo due to the distinct strengths and weaknesses of each combo.
Is Ryze overpowered right now? Of course he is, but most reworks and releases don't come out balanced, and we wouldn't get reworks or releases if Riot tried to make them balanced on release. The important part of his rework is not that he is balanced but that he can be balanced in the future.