If it's the company's "design vision" to make content scale in difficulty due to people's geographical location, then that is a huge flaw in their "design vision".
On top of that, everyone does experience this.
Don't agree? Try this for me:
Run in a straight line (do not stop), call your mount while running (or cast Esuna/Leeches), and tell me how much the bar fills before it interrupts.
Does it interrupt the moment you hit the button? No? Then you experience it.
The further along the cast bar you get, the further away from the servers you are. Even someone living next door to the servers will not be able to see it interrupt the moment they press the button, every single time. This is a design flaw.
I can tell you now, that because I am in Australia with approx 250ms, that the cast bar practically fills up nearly every time before I get interrupted. This is a whole 700-1000ms delay.
Yes the issue is related to ping, but it also related to the netcode, and how the client/server talk to each other. People with less ping will notice it less of course, but they will still notice it due to the poor netcode and check intervals. Currently there are far too many strict server checks that just don't need to be in there, and this is what is causing people's frustrations. Pushing a lot of the checks to client side (which would probably involve spending a bit more work on security measures for hacks), would practically remove this whole issue people have been having problems with.
Everyone, aside from a few, that are raising this issue on the forums are not asking for a difficulty nerf, we are simply asking for a level playing field with client side checks. I will also let you know that I have 8/9 relics, so I can obviously beat Titan. People's issues with this problem are far from the difficulty of specific fights (because lets be frank, it's not really that difficult) - it's simply the latency/randomness whether or not we will dodge something or not.