The ironic thing about this is that you say that these two would get wrecked in OU and Ubers, yet they were both banned from OU due to being too powerful. To be completely honest, I think it's because you're underestimating them or you aren't using them right. I'll cover these both separately since they both involve their own strategy.
*Useful for: Massive coverage, Protean, revenge killing capabilities, raw power*
I have absolutely no idea how you thought this had poor coverage. Greninja has access to fantastic coverage - it can't cover everything in four moveslots, but this allows it to pick and choose what it beats and adds to its unpredictability. It has Hydro Pump, Dark Pulse, Ice Beam, Hidden Power, Gunk Shot, Extrasensory, Low Kick and more in its arsenal, and thanks to Protean, it receives STAB on all of them meaning it is not only speedy but powerful. Gunk Shot is a move Greninja got in ORAS, and it's the move that eliminated Fairies as a check to Greninja, which made it very difficult to stop since it has almost perfect coverage of the game now. Another common answer, Chansey, also has to deal with the occasional Low Kick.
Greninja is a pretty simple Pokemon to use - it functions as a mid-late game sweeper and revenge killer since it's so fast and can hit many Pokemon for super effective damage. Some ideas as to how to use it:
- Use Protean to change your type to resist incoming attacks. Protean is an amazing ability because it means it can counter so much more whilst taking less damage. For example, Gunk Shot will often fail to KO Fairies, and Greninja would often lose to Moonblast or another Fairy move due to its Dark typing. Thanks to Protean, however, you gain STAB to deal more damage and your type changes to Poison, which means you instead resist Moonblast. You can use the type changing ability to play mind games with your opponent - for example, if your strongest attacks barely fail to KO, first use an attack that will cause you to resist the incoming attack, then use the strongest attack which will now hopefully be in KO range.
- If you don't think Greninja will KO and will die to the next hit, why are you keeping it in the battle? Unless letting Greninja die puts you in a better position in the game, you have other Pokemon to switch into to protect Greninja. You can also support Greninja by weakening Pokemon to a Pokemon where Greninja can KO, possibly through the use of entry hazards like Spikes or Stealth Rock.
- The standard set is Hydro Pump, Gunk Shot, Ice Beam and Hidden Power Fire with EVs of 40 Atk / 216 SpA / 252 Spe with Naive Nature. If you aren't using this, it's highly recommended.
Because Greninja is so fast, so powerful unboosted and has almost nothing in the ways of defensive counters, it's a Pokemon that was obviously toxic to the OU game and was suspect tested by Smogon. Nearly all the votes were in favour of the ban to Ubers. Since Greninja was banned it's been while since I've played with it, if anyone else thinks of anything else comment or edit :)
*Useful for: STAB coverage, sweeping*
This has been Ubers ever since it got its Hidden Ability, and is showing no signs of returning to OU at all. Speed Boost is a ridiculously overpowered ability that makes Blaziken substantially faster by simply using it for a turn. If you can find an opportunity to set up Swords Dance, Blaziken is a lethal sweeper that is far too good for OU. Most players would laugh at you if you told them Blaziken got wrecked in OU.
- A general rule with sweepers is that there is no point trying to set up if your opponent can beat you with one of their Pokemon. For example, if you opponent has an Azumarill, you would not try to set up and sweep because Azumarill can switch in/ revenge kill you at no cost thanks to Aqua Jet. Only attempt to sweep a team if you know you can pull it off.
- Know when it's safe to use Swords Dance. In your post you sound like you think it's incredibly difficult to set up a Swords Dance, but really it isn't. Blaziken has good Fire + Fighting STAB coverage that forces many switches - this would be a good time to set up. You could also set up while the opponent uses a passive move like Stealth Rock, Defog, Roost, etc. Obviously you can't just click Swords Dance whenever you want to - you just need a bit of common sense and a plan as to how you are going to find an opportunity to get a Swords Dance up.
- Like Greninja, Blaziken is frail so you should protect it from Pokemon it loses to as you would with any other Pokemon.
Blaziken will likely remain in Ubers for a long time unless a dramatic shift in the game occurs that causes Blaziken to be less viable. It is a fantastic Pokemon and it's far too good for OU, let alone UU.
My best advice for using these Pokemon and for competitive battles In general is to practise. You will learn what these Pokemon can and can't do and you will learn how to play them to their strengths. It's possible that people are having more success with these Pokemon because they're simply more experienced - if you get to know the tier/ metagame you're playing more, you'll play better in general.
tl;dr: these Pokemon are much better than you think - either you're underestimating them, or you aren't playing with them correctly.