I can add, that when items have a fixed stat like +50 attack and +50 defense, it's OFC going to be bad up until a very high level.
That's why I think % is better, because it can adjust to lower levels, and get better and better as higher levels go. Yes, at very high levels, it will be super strong. And at a lower level, they still won't always make sense.
But the flat rate will always mean they're just bad until they make sense. And at the current prices, that level is extremely high.