Vulnerable would've been fine if:
1/ Marked shot wasnt tied to a RNG proc of auto-attacks and the cast of a focus builder. That means, give Marked Shot the CD of the duration of Vulnerable window (for example, I know this is bad for AoE) and call it a day.
2/ Aimed Shot was either instant or castable in movement. Any movement inside the vulnerable window atm kills the cycle of "Arcane Shot - AiS - AiS" which is the bread and butter of the build.
Also I fail to realize where the complexity of the spec lies, other than handling movement. (that is, delaying vulnerable windows until you can stop moving).
There're 2 cycles that you have to master in the spec to rotate properly:
Windurst -> AS -> AS -> AiS -> AiS
MS -> AS -> AiS -> AiS
That's not particullary hard, tbf.
I also like a lot that AiS has 2 charges in BfA. That means you can choose when to use it as long as you don't cap charges and it will not be a DPS loss, so you can handle movement a lot better.