I think you are getting the wires crossed. Double Down, by itself, in an absolute vacuum, is fine. It's when you take into account that you are required to have three cartridges before your burst every minute due to the cost of Double Down that slows down the flow of the job that did not exist in ShB.
An example that I gave before that I will give again. In a dungeon, on the mob pack(s) before the boss, you are required to have at least 2-3 cartridges before the last mob dies, or you will either A. Be behind the buff windows of your group, or B. Burst without Double Down or Gnashing. Both suck. This means that while you are AoEing a mob pack, instead of doing what's damage optimal, which is using your gauge to help burn down the mobs faster either through Gnashing Fang or Fated Circle, you have to focus on building gauge to not miss out on the first burst in on the boss.
This is an EW exclusive issue, as in ShB you only required one cartridge to burst for Gnashing Fang at a minimum, which you could easily build in 3GCD's and be well within the burst window. This is not a dungeon exclusive thing either, you have to do this downshift in the add phases of both EX trials instead of being able to spend it like you would have been able to do in ShB. This is made even more apparent on fights with significant downtime, forcing you to hold gauge from a minute ago, in order to not disrupt your next burst.
You can argue that it's not that bad, the end result is that this change wasn't a quality of life upgrade, but a downgrade. You can argue it's a "skill issue", but I and others see it as an annoyance. Even doing something as simple as changing Double Down to a 1 cartridge cost would be a massive improvement that would not affect those who enjoy this current iteration.
As far as the No Mercy burst, the issue is NOT with Double Down, it's with Hypervelocity and the No Mercy duration. In ShB, you could end a No Mercy on a Burst Strike, no problem. Now though, if your last GCD is a Burst Strike, the following Hypervelocity falls out of the No Mercy window because there is not enough time for a follow on oGCD. Something easily fixed with a short 2sec increase.