I'll get right to the point: the change to Magma Armor with U40 where you can no longer generate ultimate while this skill is active (12 seconds) is an over-tuned nerf.
Now, I do understand WHY they made this change. Basically, prior to U40, you could hit Magma Armor (Corrosive most of the time) and then generate enough ultimate (with the right build) that you could hit your back bar ULTI while your Magma Armor was still active. This essentially allowed you to stack ultimates, which should virtually NEVER be allowed to happen.
Your solution, however, is too extreme in the other direction. Because now, while Magma Armor is going, you lose ultimate generation for 12 seconds. That is a LOT of ultimate to lose out on! And it basically makes this ultimate out of line compared to other ultimates of the same or similar type, since they still allow the caster to generate ultimate. (Gibbering Shelter, for instance, still lets you gain ulti while active.)
So, what I propose is that you instead add a COOLDOWN to ALL ultimates. Make it 12 seconds. A "Global Ultimate Cooldown" if you will. This will basically prevent ANY build from ever being able to stack ultimates. Even future builds that may be developed based on future game updates. OR, if not a cooldown, then make it impossible to stack ultimates. Meaning, if you have an ultimate going, and then cast another ultimate in the same time frame, the first ultimate DISAPPEARS. Much like how Healing Springs works, or other similar skills.
So those are my two suggestions on ways to fix this.
Making it so DK is not able to gain Ulti while Magma is burning has gone too far the other direction.
P.S. I am not a player who has ever stacked ultimates like this. I don't even play DK very much. But I noticed this change and how it affects PvP and I disagree with how heavily it got nerfed.