The IG-11 droid on the Mandalorian was designed to be an incredibly efficient warrior able to take on the most dangerous of bounties. It has overwhelming damage output because its swiveling lenses and body allow it to independently aim two blasters to rapidly unleash a torrent of accurate gunfire across a 360 degree arc. In the below GIF we see it taking on a small army of mercenaries attacking from all directions.
In addition, its Durasteel frame was highly resistant to blaster fire and it was able to survive multiple shots from a heavy cannon (pictured below) with seemingly negligible damage.
All in all, the IG droid would drastically outclass all but the most talented or Force-sensitive organics, and are more than a match for stormtroopers or the Trade Federation's B1 battle droid. Why were IGs never used for war?