Most attacks can hit aether, but actually getting to aether is the hard part. The quickest way to get to aether is above and slightly in front of it. The issue with hitting aether is aether digging deep into stages like BF and SV, then moving away once it actually comes up, making it much harder to hit without great ground/aerial speed and a very fast disjointed aerial.
Ike's aerials have enough range to just jump at and fair or bair, but on stages like SV, it becomes very difficult because of the amount of distance needed to be transgressed in those ~23 frames from when Ike jumps up for the sword to actually the spiking down. 23 frames seems like a lot, but you have to run up to aether, jump, and input your own aerial while not getting hit yourself. It's even harder on BF.
Dolphin slashing aether isn't nearly as much as a threat as merely fairing it (with marth). Spikes are dumb because we can just SDI and tech, and it's MUCH harder to feasibly pull off. MK is fast enough to use just about any of his aerials except nair. On BF though, everything becomes much harder for these characters to hit us.
gist: Having a move that can possibly hit aether =/= being quick and fast/good enough to hit it spaced well in a match.