Scale armour tends to use plates that are a lot thinner than solid plate armour (c. 0.5mm was typical while plate armour was usually over 1mm), otherwise it becomes too heavy to bear. Plus there are a lot of weaknesses introduced because the scales have to be attached to the backing (every lacing hole is a weak point). Lamellar is a lot more efficient but the lacing is still a huge problem. The mail-and-plates construction was developed as a replacement for lamellar and did away with a lot of problems associated with lamellar lacing. If you want flexibility then use mail. It is lighter and just as protective as scale and lamellar. The problem with mail is that it is the most expensive and labour-intensive type of armour ever developed. The ideal armour is solid plate as the primary defence with mail protecting the areas that can’t be covered with plate. But solid plate has to be carefullly tailored to fit properly and requires a lot of skill.In other words, the life of an Infernal Guard, or indeed common Chaos Dwarf warrior, might be quite lice-ridden and stinking, particularly when trekking the vast distances of the Dark Lands or when posted far, far away.
Sakakibara Kozan’s Chukokatchu Seisakuben presents a good summary of some of the problems with scale and lamellar - problems that re-enactors usually never get to experience.
"When soaked with water the armour becomes very heavy and cannot be quickly dried; so that in summer it is oppressive and in winter liable to freeze. Moreover, no amount of washing will completely free the lacing from any mud or blood which may have penetrated it, and on long and distant campaigns it becomes evil-smelling and overrun by ants and lice, with consequent ill effects on the health of the wearer."
The following passage from the Arabic Nihayat al-Su’l wa’l Umniyaya fi Ta’lim A’mal al-Furusiyya supports this.
"Every day he must train himself to dismount elegantly so that he does not break or damage it
All metal armours are highly protective. The problems with scale have already been outlined but have nothing to do with protective capacity.