Also you didn't calc vs. stuff like FC POgre/Tina.
InfernapeTropius11 here are some calcs that can make you change your opinion:
+2 252+ Atk Groudon-Primal Precipice Blades vs. 252 HP / 252+ Def Fur Coat Kyogre-Primal: 213-252 (52.7 - 62.3%) -- guaranteed 2HKO vs fur coat Pogre. And if don carries Bolt Strike, there's a large chance to finnish the work, even non boosted by stakeout anymore: 252+ Atk Groudon-Primal Bolt Strike vs. 252 HP / 252+ Def Fur Coat Kyogre-Primal: 154-182 (38.1 - 45%) -- guaranteed 3HKO.
+2 252+ Atk Groudon-Primal Precipice Blades vs. 252 HP / 252+ Def Fur Coat Giratina: 175-207 (34.7 - 41%) -- guaranteed 3HKO vs FC tina.
This ability is ridiculously OP because you can break or almost break any wall. Once again, watch out for the next coverage move don can use: 252- SpA Groudon-Primal Fleur Cannon vs. 252 HP / 252 SpD Giratina: 198-234 (39.2 - 46.4%) -- guaranteed 3HKO. Beats your giratina.
Stakeout seems like somewhat of a prediction game too because you only beat tina/zygc if you catch them with IB/Fleur Cannon on the switch
That's precisely what Stakeout Groudons of high ladder players do. If your opponent predicts well, you have no safe switch ins. And even if you switch to a Solgaleo predicting a Fleur Cannon (you need to have already scout the move, so prob your FC dragon has taken one hit), if the Groudon is not Choice Band/Scarf, he can use its ground move vs it because its faster.
Be aware that the calcs are for Groudon, but you have MMY and MMX as two more Stakeout Weapons:
+2 252+ Atk Mewtwo-Mega-X Close Combat vs. 252 HP / 252+ Def Fur Coat Kyogre-Primal: 222-262 (54.9 - 64.8%) -- guaranteed 2HKO. Add a potential (and almost certain) Fusion Bolt/Bolt Strike as coverage move and you can guess what happens.
+2 252+ Atk Choice Band Mewtwo-Mega-X Ice Hammer vs. 252 HP / 252 Def Fur Coat Giratina: 334-394 (66.2 - 78.1%) -- guaranteed 2HKO. The next hit is at +0 and we have the following damage: 252+ Atk Choice Band Mewtwo-Mega-X Ice Hammer vs. 252 HP / 252 Def Fur Coat Giratina: 168-198 (33.3 - 39.2%) -- guaranteed 3HKO. This means that if you stay with tina you die, and if you switch, you heavilly damage other of your pokémon.
+2 252 SpA Mewtwo-Mega-Y Psystrike vs. 252 HP / 252+ Def Fur Coat Kyogre-Primal: 171-202 (42.3 - 50%) -- 0.4% chance to 2HKO
+2 252 SpA Mewtwo-Mega-Y Moongeist Beam vs. 252 HP / 252+ SpD Giratina: 374-442 (74.2 - 87.6%) -- guaranteed 2HKO
+2 252 SpA Mewtwo-Mega-Y Psycho Boost vs. 252 HP / 252+ SpD Giratina: 393-463 (77.9 - 91.8%) -- guaranteed 2HKO
And for the specs Stakeout MMY, the new fashion toy these days, it can OHKO 252+ tina with a non STAB move LOL:
+2 252 SpA Choice Specs Mewtwo-Mega-Y Moongeist Beam vs. 252 HP / 252+ SpD Giratina: 562-662 (111.5 - 131.3%) -- guaranteed OHKO
+2 252 SpA Choice Specs Mewtwo-Mega-Y Psycho Boost vs. 252 HP / 252+ SpD Giratina: 589-694 (116.8 - 137.6%) -- guaranteed OHKO
The amount of damage, even if the move isn't supereffective or STAB, is still huge to be healthy to the metagame. Stakeout is so centralizing that you normally need to run 1 or 2 mons to check it. And as I said previously, you don't know what kind of coverage moves does your opponent runs while you don't check it. And when you check, meaning you take one surprising move, it may be too late...