He is legitimately a hard champion to play right now. It's hard to imagine that the spam-lord of button smashing is hard to play, but he is. You have to always keep your passive in mind which is hard to do under pressure, but nearly all of his power comes from his passive's uptime. His early game is EXTREMELY risky as he is very mana hungry if he is fighting/trading with lane opponents, he has no inherent escapes, and he is fairly squishy pre-6. He needs items to be effective, namely his first two items in RoA and Tear (Archangel's/Seraph).
Now that is all about the laning phase primarily. But he isn't really easy to play in teamfights either. His positioning is very tricky as he is low-midrange, pretty squishy (even with 2500HP + shield), but you have to be able to be aggressive during your passive's uptime to make the most of your damage. Now that his Q is a skillshot and his main source of targeted damage, it makes his positioning all the more important. ANY CC can really hinder Ryze as he needs to be moving and throwing out his spam combos to make the most of his kit; slows, roots, stuns and silences are bad for any champion, but Ryze I feel is extremely weak to them. In most games, I end up building Mercs on him just for this reason.
With all of that said. I think Ryze is actually REALLY strong and borderline broken in the right hands. His ability to output DPS rivals mages like Cassiopeia and Azir albeit a lot less safely. He is easily countered, but if he gets ahead he will be fairly tanky thanks to his Spellvamp and shield uptime (plus Seraph's shield) and he will be able to carry teamfights by locking down opponents with his targeted root chain and spellslinging "wqeqwq" aids.
This is just my opinion of course. Some people think he's really bad, but I feel he is just played poorly. Runes and Builds could be something to do with it, but I doubt it is a big difference.