It's all about expected return for what you are paying. I agree that Horford on the max is not getting the same return as say, Lebron, Durant, Curry, Leonard etc. on the max. I also agree that he's flawed as a player, not scoring much outside the system and rebounding poorly.
The reason I don't mind his deal as much as I do Thomas' next one is because:
1) He was an outside FA, so we got him for 4 years instead of 5 and at less than the home team max.
2) His game isn't dependent on athleticism and he should age well. I don't think there will be a huge difference between him at 33 (his last year of the deal) and right now (almost 31).
3) There are no viable alternatives to him in the draft or in free agency.