A -20 penalty seems too little to me: an arm broken not only prevents the use of 2 handed weapons, but also makes truly hard each task which requires the use of 2 hands and/or balance (and combat requires both)... I would give a -50 penalty to unarmed combat OB
-50 is a penalty that many criticals mention for a broken leg or a severed muscle in the leg. And
that IMO makes combat much harder than "just" a broken arm. It might be that a slightly higher penalty than -20 would also be appropriate, but -50 seems way too high to me. Of course it is up to you to make the rules for your group.
and some penalty (like -25 ) even to 1 handed combat OB: if you have an harm broken and you move around swinging your weapon with the other the pain from the wound will make you see the stars...
I agree that such an injury should normally have a negative impact also on combat with a one-handed weapon wielded in the other hand. But for one-handed weapons we still stick to the normal rules. So if the critical description does not mention an activity penalty we don't add one in that case. Not being able to use the shield or a two-handed weapon might then be the only penalty.
We assume that martial arts combat involves the use of both arms and legs for attacks. If one limb is missing that makes attacking harder. For one-handed weapons we assume that only the hand wielding the weapon is used for attacks. The other arm is then not used for attacks and can, kind of, be made useless without negative impact on the attacks. Of course, depending on the exact injury, the pain might lead to an activity penalty. But if the critical does not indicate this, then we don't add such a penalty - and I simply don't want to adjust every critical that is missing such a penalty.