Here is some REALLY convenient info regarding shield pressure, especially spacee pressure:
Now, I'd like you to notice something wrt Falco. Even when he performs the latest dair possible to achieve the tightest dairshine he can, there are still two frames during his L-cancel lag where you are not in shieldstun. These 2 frames usually are not enough to do anything other than roll or spot-dodge for most characters. Even the spacees, with their 1-frame Shines, can't use Shine OoS to escape this pressure before the shine. Samus, however, CAN. Keep in mind that upB OoS is just a JC upB. This means that Samus must go through only one frame of jumpsquat before she can cancel the jumpsquat into upB. The upB has invincibility on frame 1, which lasts until frame 5, and its attacking hitbox comes out on frame 4. In other words, as long Samus can start
her upB, she's guaranteed to get an invincible hitbox
out that carries over seamlessly from her frame-1 invincibility.
The implications of this are tremendous for Samus. It means that, even in the two frames of no shield stun in Falco's tightest form of dairshine pressure, Samus can still
upB out, using one frame for jumpsquat and the other frame to start the upB. Once the upB is started, it will probably beat out anything Falco does, trading at worst if performed well. This is why Samus has one of the BEST OoS games in all of Melee. Nothing truly keeps her trapped in her shield. If she wants to get out, she WILL get out. As you can surmise, if Samus can upB out of late dair > shine, then she can probably get out of any
of the 'early-late' traps with upB. The timing is sitll tight, though, which is why I mentioned the leniency involved. A lot of times, it may 'feel' like you're stuck in shield during a certain fast shield string because you're (understandably) missing those couple of frames where you can get out.
Now, upB's hitbox and frame data is perfect for ensuring beat-outs or trades, but in my experience, I find that one of the best ways to hit the spacees, especially Fox, OoS with upB is to actually trade
with it on their aerial. Why? Well, the spacees get flung up in a good bit of hitstun from even just the first hit of upB. However, Samus usually can't take advantage of it because she is going through the entire Screw Attack motions herself. By trading, Samus gets knocked out of the upB animation early with minimal KB and the spacee is still flung up into the air, leaving Samus completely free to follow up.
I personally actually aim for trades for this reason sometimes when I know the opponent is going to dairshine and it works!
If it's Fox, Samus gets knocked lightly out of upB and has all the time in the world to retaliate. If it's Falco, at low %'s, it's the same as with Fox but with a little more hitstun for Samus, and at mid-high %'s, you just have to make sure you tech the ground. This 'cancelled upB' combos into pretty much anything
way more reliably than the full upB does.