Being a pseudo legendary doesn't automatically make something good. Metagross, who is also a pseudo legend, dropped to UU this gen despite being solidly OU since it was introduced because of Steel losing resistance to Ghost and Dark (which I still think was a dumb move on Game Freak's part). Haxorus and Hydreigon are stuck in BL because of their frailty and Fairies (Hydreigon also wishes it had Nasty Plot). I think Goodra's problem is its lack of good recovery and that pure Dragon typing is really meh. If it had Recover or something, it would make a great special wall. It could also use a little bit more power too.
And hell, being legendary doesn't make a Pokemon automatically good either: see Articuno, Regigigas, etc.
He's right, being a legendary or pseudo-legendary doesn't always mean you'll be guaranteed as a higher tier Pokemon, and of course there are flaws with these guys that 6th Gen caused for them due to the aforementioned changes.
Haxorus and Hydreigon struggle very badly against Fairy-types if they don't have the right coverage move equipped plus they're fairly frail against them thanks to the former's weak Sp. Defense and the latter's nasty 4x weakness to Fairy moves, meaning one of those lil cuties pretty much spells easy death for the tri-headed dragon. Gardevoir, Azumarill, Sylveon, Clefable and Togekiss run rampant and they have the power and stats to back them up.
If Goodra had a little more in its offensive stats it would have been higher up, but yeah it's the lack of recovery that brings it down because in most situations because Rest is not really considered "reliable". I found it weird how something based around slime and goo couldn't learn Recover, but still the Assault Vest makes it quite the wall, and Gooey can make physical attackers think twice.
As for Metagross... this shows just how much the loss of Steel-type's resistance to Dark and Ghost brought down such a powerful beast, and it was made even worse by the fact that Meteor Mash got weakened in base power meaning it's not as reliable as it was previously. However, this was a blessing to Bisharp, who still remains the sole Steel-type to resist both Dark and Ghost. I found this nerfing just as much of an impact than the Fairy-type.
Over the generations we can really see once previous beasts now becoming so bad in the standard game, Articuno was quite a powerful Ice-type back in 1st Gen days but the introduction of Stealth Rock really made the icy bird fall from his former glory. Or how amazing of a wall Snorlax was in the early generations but then the dominance of Fighting-types just meant he couldn't survive in an environment full of these brutes.
And given the new Megas we'll be seeing in Omega Ruby & Alpha Sapphire I'm imagining even more changes when those games come out, I know for sure Wash Rotom may have to run Hidden Power Ice due to Mega Sceptile heavily resisting its STAB moves. It's like the day we saw Scizor really get his claim to fame when Platinum gifted him with Bullet Punch.
And as said by Tamashii Hiroka herself in one of her videos (Top 5 Worst Legendaries) being Legendary doesn't always mean Uber.