I think the in-house play testing was heavily skew to the point that a lot of the things the the community saw as UP or OP was fine there.I'm going to have to come down to saying that the imbalance is intentional.
I don't know that the Next design team intended to make imbalanced classes as such.
Nevertheless, they intentionally made spellcasting scale from sleep to hypnotic pattern to wall of stone, with a vast array of specified discrete combat and non-combat options, and martial capabilities not really scale apart from more damage, in the "front 10" levels alone (with a few exceptions such as Battle master or hunter ranger tricks), with a trickle of defined non-combat functionality outside of spellcasting (mostly confined to Expertise for bards and rogues, Natural Explorer for rangers, and a few other tricks for rogues), never mind the "back 10" when we get stuff like planar ally or wish versus... your third use of Indomitable in a day or a third die on Brutal Critical! (Whew!)
They made those design decisions, and I don't think they were so dense as to be unable to imagine what the consequences would be. So, yes, intentional.
That's the true heel of 5e, the design team didn't have enough diversity. Diversity of playstyle. And they let people with similar playstyles blind them to other styles.
That's why 5e is chock full of "Why does this work like that." and the team being shocked what people played.