I get that he has moved up in relativity due to some others getting toned down AND in objectivity from getting access to Tremor who flawlessly complements his game plan; but again, was it really THAT drastic? I'm seeing people go as far as top 3 for him which is wild in my mind. How are Peacemaker, Kenshi, Liu, Johnny, Raiden, Sindel, Ashrah, Tanya, and Ermac not all higher than him, putting him at the very bottom of top 10 at best? I'd also argue Kitana, post-buff Quan, and post-buff Rain edge him out on average for general power level & matchups.
Reiko has always been underrated. The character's "flaw" has been that he does not particularly synergize well with the game's former (i.e., Cyrax and Stryker) and current (i.e, Khameleon) top tier kameos. However, Reiko remains very strong because he has the best mid-range low poke, which on hit leads to some of the best offense in the game (safe overhead, advantageous sweep, and launching command throw). He also has the best projectile in the game by far, excellent comeback potential with command throw and fatal blow combos, and a couple of long-range buttons that are ideal for whiff punishment.
Top tier characters and/or their respective top tier kameos have been getting nerfs, resulting in unintended consequences for some of the mid and low tier characters. Reiko essentially received a massive buff when Tremor was released. Subsequently, Tremor himself received a couple of significant buffs. Nobody ought to be surprised that a character like Reiko, who continues to be unchanged, has benefited from this environment.
Despite their buffs, I have seen no evidence in tournaments thus far that either Quan Chi or Rain is better than Reiko. For what my opinion is worth, if I competed in this game, which I would not because I consider the meta stale, I would definitely use Reiko/Tremor. The only characters who may be better are Ashrah, Kenshi, Sindel, and Tanya. Ermac is too new for me to comment.