Warhammers > Full Plate
There are lots of modern depictions of how people in full plate are much more maneuverable than we used to give them credit for, but there's also a drastic misunderstanding about how incredibly durable people in full plate are. There is a lot of metal and cushion protecting the wearer from impact which renders them almost impervious to single decisive stikes against all but their most vulnerable regions (i.e. heads and joints) which they will have learned to guard more effectively.
Enter the Warhammer. What better weapon to destroy armor than the tool used to create it? I do not know of a better force multiplying tool for getting through heavy armor than a Warhammer, and that is because...
Warhammers are NOT Blunt
If you're like me when you hear someone say "warhammer," you think of a big oversized sledge hammer that probably has big square hammer heads. That's highly impractical for a number of reasons if you think about it, which is why this is a better idea of what a typical warhammer would have looked like:
Note that the head is very small so that you can focus all the momentum of the hammer on a tight little point. Note, too, how the back end of the hammer is a long spike for punching through the armor in its weakest spots.
I understand why most fantasy settings don't depict warhammers this way because of the rule of cool, but sometimes practicality should take over. This is particularly true when talking about a weapons weaknesses, speaking of which:
Warhammers are Slow
Now a good warhammer is actually only around 4 pounds, but that's actually fairly heavy for medieval weaponry, and you actually want it to be a little unbalanced specifically because you want all that force focused on the end.
What this leads to is a big, awkward weapon to weild in close quarters, especially against a lightly armored opponent. That's not to say a warhammer won't kill an unarmored target more easily than an armored one. Any hits on an unarmored foe are likely to be debilitating, if not fatal. However, while a warhammer regiment will have to make slow, accurate strikes and maybe worry about getting their spike stuck in people's fleshy parts, a team with lighter and more agile swords will make quick work out of anyone they can actually kill with a blade. This is why most actual knights carried multiple weapons for a variety of circumstances.
Thankfully, your hammer regiment is already heavily armored. What this means is that they are significantly less threatened by lightly armored opponents. They can afford to get inside the reach of lightly armored units and engage with less leverage, knowing that their armor will protect them unless they lose their balance (and even sometimes after they lose their balance).
What you end up with is not so much a group that is bad at killing anyone, just a group that might do better if they diversified their weapon set a bit.