Sure, I'm late.
In my experience, most SFIV Ultras are going to get blocked unless your opponent has committed to a super slow button.
They do not operate identically to Ultras in SF4. Ultras have startup AFTER the flash, not DURING.
There are some zero-frame-post-flash Ultras and a lot of zero-frame supers in SF4, that you can't block or avoid if you were in a position to be hit when the flash happened - for example, Zangief's UFAB is not jumpable or counter-super-able after the flash even if you were just standing next to him, and most Supers are not blockable post-flash if you were not already holding Back. The
superflash itself is still just aesthetic; the game allows your character to interpolate their animation from "last frame" to "this frame" over the course of the superflash because it looks cool.
This is why SG has the "Skip Superflashes" option in training mode, to show they are just for style and don't impact gameplay.
The way Rising Thunder does it means the superflash
is no longer purely aesthetic...it is not possible to HAVE a zero-frame super, where your opponent is not able to avoid it if they weren't airborne/blocking appropriately before the flash. And the big problem with that is that a superflash is much easier to react to than a DP.
For example, if you have a 4f DP, the opponent has 4f of real time to react. They're going to get hit unless they were anticipating it.
If you have a super that (let's say) allows the opponent to move for 4f of "game time" during the superflash, but the superflash is 120f long, that's 2 seconds of REAL TIME that the other player is given to react, even if the character only advances 4f of game-time, because as long as they are holding back by the end of that 120f they'll block it. So in effect they have 120f to react to your super, compared to 4f to react to your DP. NOBODY will get hit by that super unless it was a real punish. And considering supers cost meter, this is at the very least counterintuitive to players, because it makes supers mostly useless as a neutral-game tool.
[edit]
It also makes comboing into supers frustrating, because people are used to the superflash implying that if the opponent was in a position to get juggled by the super when the flash went off, it will properly juggle. In RT, since you can SEE THEM LAND during the superflash when you are powerless to do anything, it FEELS worse. The difference between an Ultra with 7f post-flash startup that whiffs as a juggle and an RT super with 7f during-flash startup is that the during-flash startup feels like it's the GAME's fault you missed, whereas the post-flash startup feels like it's YOUR fault.