Currently, PTR 3.22 unbreakable armour gives +25% armour and the glyph takes it to +30% (tooltip on glyph is a bit confusing).
If you compare that to bone shield at 15 seconds, giving a flat 20% reduction on top of your normal tank armour, I find that bone shield is ahead on total damage reduction. I modelled this in a google spreadsheet.
I used example armour of 25123 unbuffed and 27858 buffed.
Unbuffed, UA leaves you taking 32.66% of the damage on you, whereas bone shield on your armour leaves 30.20%. Smaller numbers are better. Glyphed, UA is 31.18% meaning bone shield is still ahead.
I think UA glyph should be changed to approximately 5% flat damage reduction, and the UA duration normalised to 15 seconds.
Vampiric Blood and bone shield are already 15s when glyphed.
This proposal is on the USA wow forums.
Another factor is that UA does not multiply the armour from buffs. Source. This means a raid-buffed tank gains less than a 5-man tank.
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