Hashmal is a weird one.
On an individual level, the mechanics aren't hard at all, it's entirely an attentiveness check. Has he just jumped? Be nearish the middle and ready to run to the safe side. ControlTower? Be near your marker. People who can't handle these things with some vague semblence of consistency shouldn't even be in the instance if you ask me =/
In honesty, the ground markers after earth hammer leading into adds aren't especially problematic, if the group wipes there, they deserve to as you generally only have yourself to blame if you die at that point.
A tip: if you get trapped, just hang on the edge and find a single non stacked marker to eat, it's not pretty but you'll survive, just be sure to get back in range of your healer afterwards to get topped up before the golem aoe starts hitting hard.
The 3 stack markers during the orbs are more annoying as it's easily to get wiped through no fault of your own and that's likely the only thing I'd change about the encounter TBH.
Another Tip: This is also very survivable with calm heads. If you get the stack marker, do the right thing for your team and just run out with it IMO. Alternatively, if the markers are on other people and they aren't moving around, angle yourself on the edge of one of the markers to the opposite side of the other, 4 people will survive the stack with barriers (and maybe even without) np and it's entirely possible to get through unscathed as long as both markers aren't stacked tight.
Honestly, the thing that annoys me the most is when you see people repeatedly run into the unsafe edge for the dive, I can often save 2 people from that mechanic if Rescue is up, but not if they're not even in range =/
Frankly, the biggest problem I have with Rabanastre is that the difficulty falls off so massively after Hashal, the last 2 bosses are genuinely difficult to wipe to IMHO which leads to the sort of complacency we see on the first pair.