Let's consult the frame data! Threads referenced:
http://smashboards.com/threads/yoshi-frame-and-misc-data-thread-wip.391441/
http://smashboards.com/threads/rosalina-and-lumas-frame-data.383688/
Forward Smash / F-Smash / FSmash / Side Smash (normal)
Frame 14-14: 15.5% 30b/ 98g 361°
Frame 14-14: 14% 30b/ 97g 361°
Frame 14-14: 13% 30b/ 97g 361°
Duration: 52
Frames between last active hitbox and end of move: 38
Up Smash / UpSmash / U-Smash / Usmash
Frame 11-13: 14% 37b/95g 75°
Frame 14-16: 12% 37b/95g 75°
Duration: 46
Frames between last active hitbox and end of move: 30
Down Special(1): Yoshi Bomb (ground)
Frame 7- 7: 4% 100f/80w 80° 0.5-Hitlag Ground-Target-Only
Frame 27-28: 15%(+10) 60b/72g 80° 0.7-Hitlag
Duration: 73
Yoshi Bomb Star
Frame 3-12: 4%(+6) 50b/40g 50°
Assuming the stars activate on frame 27 (that'd be when Yoshi hits the ground), the move has active hitboxes up until frame 39.
Frames between last active hitbox and end of move: 34
Back Air / B-Air / Bair
Frame 11-11: 2.5% 30b/200g 365° 2.0-Hitlag Aerial-Target-Only
Frame 11-11: 2.5% 50b/150g 361° 2.0-Hitlag Ground-Target-Only
Frame 17-17: 2.5% 30b/200g 365° 2.0-Hitlag Aerial-Target-Only
Frame 17-17: 2.5% 50b/150g 361° 2.0-Hitlag Ground-Target-Only
Frame 25-26: 5% 30b/184g 54° 2.0-Hitlag
Enables transition to Bair landing state (landing lag) on real frame 11
Cancels transition to Bair landing state (landing lag) on real frame 44
Landing Lag: 19
Duration: 55
Frames between last active hitbox and end of move: 29
Assuming we autocancel PERFECTLY, this gets dropped to 18.
Naturally, the punish window for Bair is harder because we can move during it.
So, in conclusion, all of our "kill moves" have about 30 (usually more) frames of punishable endlag. That's at least half a second to punish Yoshi hard!
Let's look at Rosy's Usmash, since that's one of her kill moves:
U-smash
Frame 8-16: 12% 45b/1080g 86°
Smash charge window on real frame 4
Begins partial invincibility on real frame 7
End partial invincibility on real frame 17
The hitbox comes out on Frame 8. Luma's comes out on Frame 9.
So you have, on Bair, assuming a perfect auto cancel from the time of the last hitbox, 10 frames to get into position and start an Usmash. Realistically speaking, you probably have more time than this.
On Down B, you can roll away from the second hit or whatever, and then you have 34 frames to hit Yoshi after the stars vanish. That's 26 frames to run in and Usmash.
Against Usmash, you have 22 frames to start an Usmash.
Against Fsmash, you have 30 frames to start an Usmash.
Yoshi is by no means unpunishable. Look at this frame data! This SCREAMS punish me!
I'll admit to not knowing how this frame data will work out with shields. Maybe you can't punish a Bair on shield. But the rest of this stuff? I'm positive 20 (or more) frames are enough to get an Usmash in.
This is one of Yoshi's weaknesses. He struggles in last hit scenarios. He can't setup his kill moves and if he fails to connect with one he SHOULD get killed in response. The frame data just does not work out in his favor.