Im delving into Ubers/any legendaries.
Overrated:
Suicune
Even though I really, really like Suicune, I see it a lot outside of Reborn to the point where it is excessive. It might have good walling abilities, combined with the power to shut down Rain Teams with a single switch to an incoming water type attack, makes most rain users (excluding Ame because he's so boss with rain) suffer a headache. Thats pretty much what it's good for. Nothing more. CM set up? My TrollTales will take that and Psyshock you.
Rayquaza
While it can shut down weather teams with Airlock (a very fun ability) and has great coverage and speed, it simply does not have the power to stand Stealth Rocks and Ice moves. The most frequent set I see (Focus Sash, DDance, Outrage/Other Physical move here) is easily rendered pointless with the existance of Steel types not named Aggron or Bastiodon, since most Rays I see have EQ.
Underrated
Giratina
For some odd ball reason, I dont see this. At all. Giratina is probably one of the most adeptive Legends out there with a great typing that shuts down Fighting types (which are pretty common in this metagame), a cool move pool, and acess to two formes.
The Altered Forme is bulky as hell. A maximum of 502 HP combined with those high defense stats make OU walls jelly when played correctly. While lacking any decent recovery other than Rest, it can tank and recover little by little thanks to leftovers. It can also abuse status conditions thanks to having both Will-O-Wisp AND Thunder Wave, which can devastate a set up.
With the Origin forme, it swaps Defense for offense, and with the Grisious Orb+STAB Shadow Ball/Claw or Dragon Pulse/Claw can really punch a serious hole on the other team. It's not fast and cant use a scarf, but again, the HP and pretty decent defense ensures that Giratina lives through at least one unboosted Super Effective move against it.
Manaphy
No uber rain team carries this. Why, I do not know. Hydration Rest spam with FUCKING TAIL GLOW makes this little thing aboslutly terrifying! It's bulky enough with some stat arrangements to abuse this, and make short work of anything.
Hated
Genesect
I really hate Genesect. Never could point a figer as to why, but it's another OU Steel type thats abused by a choice item, namely Scarf. It's powerful and can pun a serious hole in my sun team considering that it's used late game. I have more reasons but the bell just rung so ill get back to this later