Many people seem to find around their bodyweight is the point where form breaks down - in many compound exercises. It takes real attention to technique to get beyond there with decent form. Moving more than your bodyweight on a bar with decent form is a kind of strength which takes quite some time to develop.
I would suggest that if you are losing form on a certain weight of barbell rows, you will probably lose form on dumbell rows, too. Many people turn it into a twisting exercise, which ain't good for you.
Underhand grip on the barbell row will, like an underhand grip on chinups, use your biceps more. Overhand grip will use your brachialis more. I think everyone knows where the biceps are, but the
brachialis is a bit lower on your upper arm.
The biceps attach to the radius - the lower arm bone that runs the side of your thumb up to your elbow - and help you turn your hand up. Flex your elbow, turn your palm to the ground, now turn your palm up towards you. Notice that the biceps shorten doing this; as well as being an elbow flexor, they are a forearm supinator, in fancy-talk.
The brachialis attaches to the ulna - the lower arm bone that runs the side of your little finger. So they're only an elbow flexor. They actually have greater potential strength than the biceps alone, but of course greatest overall strength comes from using them together. That's why people usually find that underhand grip chinups are easier than overhand ones.