First, all sources of damage reduction results in diminishing returns.
If you’re doing nothing but standing in town taking stuff on and off you could easily draw a very incorrect conclusion about Yangs-impact on your toughness and mistake it for “2x more toughness”. That’s not accurate in combat at all.
Yang contributes to the total RCR, which is used for the damage reduction (CC set bonus) and that 50% has a big impact on RCR, but its also a diminishing impact.
Paragon: 10% RCR
CC Set: 20% RCR
Yang: 50% RCR
NOTE: Manticore 50% CA cost red. doesn’t count towards this since its not generic RCR
Total RCR= (1-paragon (10%)) x (1- cc set(20%)) x (1-yang(50%)) = 36% of cost or 64% RCR overall.
Without the yangs, the RCR is 28% and while 64% is more than twice 28% that doesn’t translate into 2x your toughness because its just 1 source of damage reduction. A character will still have armor, resistances, abilities like vengeance, items like wraps of clarity, elusive ring, unity ring. All of these will be separate sources, and combined together have diminishing returns on the damage.
Second, sentries will auto fire MS nearly twice as often as CA
Its easy to test this, run out somewhere and drop a sentry, time it for 30 seconds, and count how many times it casts an ability. Do it with CA and then switch to MS and you’ll notice a huge difference. Each of those MS casts are getting the damage increase from Yang and DML.
The big difference comes when your character starts shooting MS. In addition to the auto-shots, the sentries will fire when you fire and since you’re firing MS twice as fast, so will they. Even when CA does twice the overall damage of MS, your effective MS damage with the faster auto-fires, firing twice as fast, hitting baddies twice when below 60% health… you’re going to be doing way more damage with MS.