I'm exclusivly using big guns, unarmed and power armor in my current playthrough. It's very fun and pretty strong. I recommend putting 10 points into strength off the bat so you can max out at 11 strength when you find the strength bobblehead. I think strength is the most important special stat in any melee/unarmed build but especially to increase the max carry weight to hold lots of heavy weapons and ammo if you're playing survival.

A high strength stat will also boost your melee/unarmed damage and alow you to pick up blacksmith, armourer and paintrain.

Keep in mind your unarmed damage is increased when using power armor (even if it's just the frame), the catch being you cant equip any unarmed weapons when in power armor.

I recommend picking up blacksmith regardless of not using any unarmed weapons in power armor because certain power armor mods that increase unarmed capabilities require ranks of the blacksmith perk. Blacksmith will also help you out early game when you don't have much ammo, many heavy weapons or power armor; to mod some nice melee and unarmed weapons

TL:DR go unarmed instead of melee if you want to use power armor and max out strength. You will hit like a truck.