I don't think heavy armour is necessarily underpowered for Act 1, but it drops off steeply by Act 2, and is backstabbed again by the best heavy armour in Act 3 requiring no proficiency.
This isn't unique to heavy armour. Itemization in this game is rather bad for many classes and/or playstyles, to an extent where a careful search for balanced mods is required to re-balance. Dex builds eventually get spoiled for armour, yes, but they struggle with *weapons* that aren't Phalar Aluve for a sweet while.
The itemization is one of the weakest aspects of the game outside the incentivised playstyles, alongside the incredibly bugginess and underpolished Act 3. If you don't want to respec every couple levels, you're going to suffer at some point.
Early on, you're essentially trading AC for better mobility and higher damage (ignoring the possibility of Hill Giant elixir addiction here). That's fine enough for Act 1, as it makes sense monks and such would be harder to hit. They're nimble. It's their *thing*. It's Act 2 where issues arise, and make no mistake, they're here because there were no thoughts spared, the same way there were no thoughts spared about how unbalanced access to so much hill Giant Strength was from level 1.
Larian has a history of being really bad at balancing, mostly through thoughtlessness and allowing exploits. It is what it is.
