Okay, I looked at them, and I'm telling you it isn't good on Shyv. It has less attack speed than any other offensive item she would want until after she's fully stacked it. It has less on-hit damage than any other on-hit item she could buy. It has AP that does nothing for her. It has AD that, while decent, is not as good for her as raw AS. And finally it has the phantom hit proc, which is less good because you don't get it until after you've fully stacked it, and then once you have it's STILL less good because Guinsoo's own on-hit proc is so weak.
Her W is only sticking power if the opponent has no CC or doesn't understand when to time their CC correctly (which admittedly is a lot of people). Her ult just catches you up to your opponent's mobility skill, which will come off cooldown before your ult will be useable again.
Even if you need to out-duel someone, you will get more value from BotRK or Bloodrazor. If you need more damage, you will buy both of those items before you will buy a Guinsoo's, and if you buy all three you will melt before you get the time to fight anybody.
No, her AP scalings are NOT a good bit of damage, because damage is (for the most part) not why you're using those skills. E's damage is negligible, especially since it's the skill you level last. You're using it to get the on-hit proc, which doesn't scale with anything but your opponent's max hp. For this reason, you're also using it at the start of a fight, before Guinsoo's has been stacked up. It then has a fairly lengthy CD. If you're opponent actually stands and fights you, good chance the fight ends before it comes back. If they run, the cast time makes casting it while chasing not something you want to do typically. Likewise, her ult is used mainly for the defensive stat boost and to close the gap. Unlike her E, the damage for her ult is at least solid, but not something you're so reliant on that it makes AP in any way desirable. Just because you have an AP scaling does not mean that you want AP, it's still a garbage stat. If it wasn't, then Rylai's/Liandry's Shyvana would probably be a thing.
The champions who actually use Guinsoo's do so because they can do do something to virtually guarantee they reach full stacks. Jax can stun you. Xin can knock you up. Kayle can hit you from range. And Yi is genuinely unkitable because he's immune to slows. Shyvana has none of those things, and thus she can't force the stacks. Yes, she can Q you to get a burst of them at the start, and then the person she hit can promptly jump away and slow her and that's as far as they'll go. Not only that, but champs who use Guinsoo's (especially Yi) typically build more offensive items than Shyvana can get away with, and so they have more space to build Guinsoo's AND something else, typically multiple something elses, because Guinsoo's typically wants other items paired with it to be strong.
You do not start a fight at max stacks with rageblade. Giving more damage at max stacks does not mean you do more damage because you need to fight for long enough to GET to max stacks, and then continue to fight for long enough for damage at max stacks to outscale. Plus something you're neglecting, Phantom Hit proc does not give a fuck about your AD.
Series of Devourer AAs goes like this: 60 > 120 > 60 > 120 > 60 > 120 > 60 > 120 > 60 > 120 = 900 total damage
Series of Rageblade AAs goes like this: 35+15 (50) > 38+15 (53) > 41+15 (56) > 44+15 (59) > 47+15 (62) > 50+15 (65) > 53+30 (83) > 53+15 (68) > 50+30 (83) > 50+15 (68) = 735 total damage
So let's see, after 10 auto attacks, Devourer has done 900 damage, while Rageblade has done 735. While the non-phantom hit proc is dealing 8 damage more thanks to the AD, the phantom hit proc is doing 37 damage LESS because because Rageblade's on-hit proc is so weak. Not only that, but those Devourer AAs all came faster because it has 40% AS by default, and Guinsoo's doesn't hit 40% until stack 5. By time Guinsoo's gets 48% AS at stack 6, the damage deficit is so large that Rageblade isn't going to outscale Devourer unless the fight goes for like 5 minutes, if it outscales it at all.
Now, you could make the argument that the AD makes up for this because it empowers her Q and W, but then AD is also weakened by armour, which more champions have and in larger amounts whereas Devourer's damage is purely magic, with MR typically being present on fewer enemies and in smaller numbers.
Then, afterall that, you take into account that Rageblade is also MORE EXPENSIVE than Devourer is, and not by a small amount, by 1200 gold. This forces her to build Cinderhulk first, which not only delays her damage spike substantially, but also delays her ability to build better tank items. For less gold than that, I could build Devourer and go straight into Deadman's Plate, giving me more damage, more tankiness, faster and for less gold than Cinderhulk into new Rageblade.
If you are seriously trying to make the argument that new Rageblade is superior to Devourer, you are just straight up wrong.