Heavy armor offers the great advantage of protection! However with great protection comes a couple of caveats (PHB 144).
First, a character should be proficient in their armor. If not, they basically cannot function. Makes sense. However, being proficient in heavy armor does not mean you are proficient in medium and light armor!
Though, proficiency is the obvious part. The other important part is a characters Strength score. Most types of Heavy Armor list a required Strength. A character CAN wear heavy armor regardless – but if they are too weak, they lose 10’ of movement. Ouch.
Pro- tip: dwarves are immune to the speed penalty (PHB 20)
DM Thought: if a proficiency for a weapon or armor makes sense for a character’s backstory, consider allowing them to be proficient at the beginning of the game, regardless of whether their class or race gives it to them