I had a look a DOA3 and every move of Helena's that's stuns now stunned back then with the exception of like a few moves. What exactly am I missing about this let's go back to DOA3 style stuns?
Somebody like Mr. Wah or VirtuaPai could definitely explain it better, but I'll give it a shot. In DOA 5, how many times do you have to stun someone to get a critical burst? Anywhere from 3 to 4 times? In DOA 3, try deep stunning the opponent that many times and you'll find that they often fall down after around two stuns. Obviously it depends on what moves you do, but you'll clearly be able to see that 3 just doesn't allow you to stun as many times in a row as 5 does.
Next, stun once in DOA 5 and launch, then stun once in DOA 3 and launch. I'm almost positive you'll see the difference in launch height. Because the launchers gave you such awesome height in DOA 3, there was no need to stun the opponent repeatedly to get max height. You could certainly do it if you wanted to, but again, your opponent is going to fall down in like 2 hits anyways, so what's the point? The stun mixup game is just an unnecessary risk in 3, but it's essential in 5.
Also, since you mentioned Helena, you should notice that she has more unholdable stuns in 3 than she does in 5 (I think). 88/22P is a limbo stun, 6P is a limbo stun, 33K is a sitdown stun that guarantees 33P, etc. The same moves might stun in both games, but they don't give you the same kinds of stuns. I wrote a guide to 3.1 Helena in the DOA 3 section that might be worth checking out if you want to compare her in both games. I don't play her anymore in 5, so I don't know what stuns she has in that game.
EDIT: Here's VirtuaPai explaining the difference much better than me: http://www.freestepdodge.com/threads/doa5-for-ps-vita.2093/page-8#post-71604