I've only been playing again for a few weeks, and even then not regularly (2-3 nights / week for a couple hours). It seems as though the number of people using this set is enormous and it has become the defacto standard these days for both stamina and magicka builds. For stamina builds it can make sense to increase survivability and as most are up close and personal in their fighting style, the procs should be regular. For magicka builds, I just don't see the value outside some very niche tanky disruptor builds (for which there should only be one per group regardless).
For those who don't recall, the bonuses from the
Black Rose set are as follows:
(2 items) Adds 1064 Max Health
(3 items) Adds 967 Max Stamina
(4 items) Adds 967 Max Stamina
(5 items) Increases the Magicka and Stamina restoration benefit from the Constitution Passive by 50% and increases Weapon and Spell Damage by 154
While wearing 5 heavy, the passives you receive are armour, magic resist, health, health regen, and block cost reduction - all survivability related. The Constitution passive also returns 930 Magicka and Stamina (based on 5 heavy) once every 4 seconds when hit. Being as you'll probably not get the proc exactly every 4 seconds just as the cool down wears off in combat, using 5 seconds for debate purposes makes more sense. That works out to 186 regen per second, or 372 regen every two seconds (how often regen ticks proc).
One can thus look at the
Black Rose set as giving you an additional regen value of 186 Magicka and 186 Stamina ONLY while you are actively fighting and getting hit.
Alternatively, if we look at the
Amberplasm set obtainable from Ruins of Mazzatun, the bonuses are as follows:
(2 items) Adds 967 Max Magicka
(3 items) Adds 688 Spell Critical
(4 items) Adds 129 Spell Damage
(5 items) Adds 300 Stamina Recovery (actually 360 if you use 5 light armour)
(5 items) Adds 300 Magicka Recovery (actually 360 if you use 5 light armour)
360 regen all the time, vs 372 regen only while being hit in combat. The better bonus seems rather obvious.
In using this set, you can also get the passives from light armour: Magicka cost reduction, Magicka Regeneration, Spell Resistance, Spell Critical Strike chance, and Spell Penetration. In all except some niche builds, much more effective than the heavy armour passives.
This set is available with jewellery, so you can use a necklace, 2 rings, and 2 pieces of armour (I'm not certain if it comes in weapons) - so you can still wear 5 heavy armour if you want the passives for survivability.
Pairing 5
Amberplasm with 5
Kagrenac's Hope (max magicka, magicka recovery, max health, 225 spell damage, and fast rez speed) in light or heavy (if you're in need of those passive - which skilled movement / play shouldn't require) seems like a much better option than for any magicka build to use Black Rose. The last item if in an 11-piece build would probably be a Molag Kena head or shoulders (for spell damage), or if in a 12-piece build then a monster set that makes sense for whatever you're doing (Valkyn, Blood Spawn, Molag Kena, or other).
Am I missing something critical in my thinking?
EDIT / UPDATE: There are some niche builds such as a defile tank (5 x Fasala + 5 x Black Rose + 2 x BloodSpawn or Malubeth) likely best run by a Magicka DK that this set is very well suited for. I still don't believe it's appropriate for the grand majority of Magicka builds who are using it.
EDIT/ UPDATE 2:
@Lexxypwns @caeliusstarbreaker (and others) have given some good thoughts and suggestions, that have prompted me to want to try this set to get a feel for it. The math behind the heavy armour passives doesn't endear me to the set, though there is a lot of subjectivity in value that will be clarified once I test it out.