Against an opponent in the air use SRSH B. Actually lands before theyre back on the ground and gives guaranteed follow-ups on both standing and airborne opponents.
Second scenario. Ivy using some horizontal unblockable against you at mid distance with you standing there in SRSH.
...
For real now. Youre just being desperate.
First. The only situation where youd be in SRSH at that distance is after 88[ B ] (SRSH is a close-range mix-up stance to be used when youre on the offensive, its not supposed to be used at any range but close except in that particular case because its the only the way to get guaranteed follow-ups after that move). Its a pretty slow move, but has big damage potential and a built-in sidestep. Naturally you'd want it to hit. Of course that aint happening all the time so sometimes youll see it blocked or whiff. After block youre at disadvantage, but have a few roulette options to get away from most punishers. Do SRSH A and you get raped, unless your opponent for some reason does an attack that is like i40 or slower.
Admittedly, UBs tend to be that speed in most cases, BUT those 2 unblockables you speak of still beat out SRSH A. The 1st one is chargeable, so releasing it early after seeing the slooooow start-up of SRSH A will interrupt the latter. The 2nd one is cancelable and a TC. The player can choose to either cancel in time and duck the SRSH A or just stand by watching with a big grin on his face. Either way SRSH A is not gonna hit.
If you want to beat out the charge UB, roulette around it. If you want to beat out the natural UB, dash forward with 6B+K and follow it up with SRSH K to go under the high or just switch to SBH right of the bat and launch his ass with SBH B (TC).
After whiff youre most likely eating guaranteed damage. Depending on the circumstances, distance and reaction time of your opponent you may get lucky and be able to cancel->block or roulette away in time, though. If you do SRSH A... youre just giving him a free ch.
Again, there can not be a niche for every move in the game. And some moves are just too bad to be used at all. SRSH A is a prime example.