The Witcher 3 is not a great game. It's a mediocre game with great writing.
The combat is simplistic and janky. Even on Death March difficulty I was able to make it through the whole game just hitting attack->dodge->attack->dodge over and over. I eventually started taking quests that were 10+ levels above me just to see if I could beat them, and it turns out I could. I think I was able to get to around 15 levels below the recommended before I finally got stuck.
Which brings me to my next point, there is usually no real reward in most quests or exploration other than seeing where the story goes. Since you are not allowed to use equipment that is higher level than you, taking on any challenge just results in getting equipment you can't use. Even worse, that equipment never becomes useful because by the time you reach a high enough level to use it, you already have Witcher gear from 4 levels ago which surpasses it. By the end of the game I had a ridiculous collection of weapons and armor that I never even used once.
Along with that, since you're stuck using weapons at your level, fighting enemies higher than you becomes aggressively tedious. The damage that you deal out becomes severely limited to the point that you will lose all of your weapon durability on a single enemy because it takes so many hits to kill. This is the same kind of naked level-gating that games like Assassin's Creed Odyssey use. You progress is artificially gated not by interesting challenges, but just by making the enemies so tedious to defeat that you'd rather not go through the hassle.
To me those factors basically defeat the appeal of exploring an open world, because there isn't any compelling gameplay or exploration to be found, just artificial level-gating and useless equipment.
The only thing that saved it for me was the writing, and even there I am referring specifically to the side quests and DLC. The main story was a snooze-fest for me. Geralt is the most boring character in the whole game world; I would have preferred to be any other character but him.
tldr, TW3 has so many flaws that I don't see how anyone can claim that its one of the greatest RPGs of all time. It doesn't mean that people can't enjoy it (I loved FF7:R even though it is a flawed game), but objectively I think it has legitimate design issues.