Typing this on a phone so bear with me if the formating or spelling is incorrect. Also after typing the power point (hehe) I kinda got tired with typing stuff for the other ones.
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My problem with Keldeo getting “suspected” is that it does not have the "complete package" like Genesect or Landorus. Depending on the team Keldeo is on (I.e. rain or sand) and particularly the item Keldeo holds, Keldeo will lack certain attributes that enable to beat certain checks, counters, and teams. Namely these attributes are power, speed, the ability to lure in and defeat specific counters, coverage, and survivability. I'll go through the reasoning of each attribute one by one.
1. Power
I can't deny that Keldeo's rain boosted Hydro Pump is one of the most powerful attacks in OU. Keldeo's claim to fame for power, however, ends there. One aspect about Keldeo's power that is different from suspects of the past is that two conditions must be meant for its power to be levels beyond the rest of OU. The first is that rain must be up on the field. The second is that it must be holding Choice Specs. The consequences of these conditions is that the "extremely high" level of power that Keldeo has can be removed from the game with an opposing weather. Yeah I know Tyranitar, hippo, ninetales don't switch in on Keldeo (no shit). But what this means is that with smart play with your own weather starter, you can remove Keldeo's power from the game by bringing in your weather starter on other Pokemon during the match AND you put much more pressure on the opponent's Politoed. If the opponent does not bring in Politoed enough, then Keldeo's power will be stifled during the match. If the Politoed user brings in Politoed more aggressivly, thus maintaining a more constant level of power for Keldeo, they run the risk of losing Politoed faster and losing the weather war.
It is important to note that a point a lot of people overlook is that ONLY Keldeo's water-STAB is boosted by the rain. Unlike suspects of the past, Keldeo's power can not be applied to the majority of its moveset and it can be removed from the game with opposing weather. Let's take Genesect and Landorus-I for example (Lando-I is more relevant). With Genesect, nothing could be done to stop the download boosts it got. You could only selectively shift (shift not stop) its power boost with careful placement of EV's on certain Pokemon with equal defensive stats (aka not a lot). Even then, Genesect could utilize either boost on its ENTIRE moveset (U-turn or special coverage), not just one STAB. In the case of Landorus-I, Sheer Force netted you a boost that could be used constantly throughout the match which can not be removed by an opponent (unless you use lol skill swap). In addition, that boost in power could be applied to its STAB and (some) coverage moves. Oh yeah, this is without being choice-locked either.
Keldeo's enhanced power: must be choice-locked, must maintain a certain weather condition, only applies to one STAB, and must give up certain types of support when running rain (more on this latter).
Without rain Keldeo's power is much tamer than other choiced and life orb users in OU (Latios, dnite, Terrakion, KUBE). It also is tamer than suspects of the past.
2. Speed (scarf set rant)
At base 108 speed, Keldeo sits at one of the top speed tiers in OU. Enhanced by Scarf, pretty much nothing can outspeed it.
First I just want to talk about its speed outside of the Scarf set. Keldeo's nice speed (outside of the scarf set) would of pushed it over the edge for me if it were not that almost every Pokemon above that speed tier commonly found in OU has a super effective STAB against it. Because of this, non-choice scarf Keldeo has a number of options to go for the revenge kill. Latios, Latias, Starmie, Alakazam, Tornadus, and Gengar are all 100% viable options in OU that can hit Keldeo hard and outspeed it at the same time (Jolteon and Espeon can but they are meh).
Now the Scarf set, barely anything can outspeed it, but without rain it really lacks in power vs defensive cores. Without rain, even specially defensive Jirachi can comfortably wall it. But that is not focusing on Scarf Keldeo's main niche: being a strong win condition vs offensive teams. Unless you lack a strong water resist and it is raining, many Pokemon can tank at least on hit from Scarf Keldeo and attempt to KO back. In addition, Scarf Keldeo can be made into a huge piece of set up bait if you force it to use its hidden power.
3. Luring (expert belt and pursuit)
Keldeo makes for an excellent lure for Celebi with Expert Belt. It can also lure in several counters to be trapped by Pursuit with ttar. However if Keldeo elects to use either of these options it misses out on both the power to break through defensive cores and speed to go through offensive teams. Without rain, expert belt Keldeo can't reliably break through sp. def jirachi. Keldeo ran with ttar really miss the power that is given by rain and is more susceptible to Pokemon tanking a hit and KO'ing back.
4. Coverage
Keldeo is the definition of "pick and choose your counters" poke since it relies and hidden power so much. Unfortunately for Keldeo and fortunately for us, Keldeo does not have one hidden power that can cover every single one of its counters. The one that can affect the widest amount of Pokemon is HP Ghost, but then you run into the problem of your strongest attack against certain Pokemon being a base 70 hidden power.
Compared to Landorus-I who could use similar luring moves which had coverage that was above 70 base power, could be boosted by sheer force (sludge wave/psychic), could gain momentum (u-turn), and actually leave no stone unturned when it came to counters (Keldeo's ebelt can't break jello or amoong), Keldeo is just sad.
5. Survivability
I think this is the second most important point to consider about Keldeo when considering it is not “suspect worthy”. Keldeo main traits for survivability are that it resists most priority and it is resistant to Stealth Rock. If you compare Keldeo to its recently “suspected” partner in crime Landorus-I, its survivability isn't optimal. The most important trait is that Keldeo is not immune to Spikes and it is not immune to sandstorm damage (every turn in sand is another SR switchin). Keldeo does not possess any immunities to gain any free switches on. Keldeo is affected by Thunder Wave and hates any forms of status that can come from bulky waters that can tank a hit from it. Lastly, Keldeo is weak to Volt Switch which means that it can a momentum loser when checked by Rotom-w and scarf Thundurus-T. (Keldeo is affected by life orb recoil unlike Lando-I but it doesn't really run it soo...)
The point is Keldeo can take a large amount of residual damage over the match (offense has spikes, sand has well sand, stall has status), which means it can be picked off by priority after enough time.
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tl;dr high power, very high speed, luring in counters. You can only pick two of the three when running Keldeo because certain conditions like your item or your weather will prevent it from having one. It cannot have the "complete package" of having all three like suspects of the past.
edit: all mentions of "banned" and "broken" have been replaced with "suspected" and "suspect worthy" respectively. Astonishingly, the point is still the same!