While I agree Kartana could rise (good speed tier with stupidly high Attack and good defense with the ability to run a couple different sets, it can break through fatter teams easily), you are misinformed about Z-Giga actually doing things.
252 Atk Kartana Bloom Doom (175 BP) vs. 252 HP / 232+ Def Toxapex: 199-235 (65.4 - 77.3%) -- guaranteed 2HKO after Black Sludge recovery
252 Atk Kartana Breakneck Blitz (200 BP) vs. 252 HP / 232+ Def Toxapex: 152-179 (50 - 58.8%) -- 74.2% chance to 2HKO after Black Sludge recovery
Do not use Z-Giga Kartana please
On to my own nom
So this mon got a lot worse between gens, it now faces competition from Tapu Koko as an electric type with a good speed tier now, but I feel like leaving it unranked is inaccurate to its viability. Life Orb 4 Attacks is outclassed, as is pretty much every other set, except for SubCM, which is probably its best set atm and is legit good. I've had a lot of success with it when laddering. People are unprepared for it since like no one uses it, and it can catch people off guard too.
Raikou @ Leftovers
Ability: Pressure
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Substitute
- Calm Mind
- Thunderbolt
- Hidden Power [Ice]
This is the set I've been using, and it works great. It works as an effective late game cleaner and wind condition. It sets up or gets a Sub on mons like Zapdos, Tapu Fini, Toxapex, etc. It has decent natural bulk, at 90/75/100, which is more than Tapu Koko and Thundurus. It also isn't weak to Stealth Rock which is everywhere right now like Thundurus is, which is another benefit it has over the other electrics. Its speed tier allows it to outspeed Mega Metagross, which is the biggest one. BoltBeam coverage lets it break through Zygarde, Lando-T, Garchomp, etc. It also has an amazing match up vs Trick Room oddly.
Some replays:
These are two games I played vs Trick Room, and it dealt with them pretty easily.
http://replay.pokemonshowdown.com/gen7ou-561869992
http://replay.pokemonshowdown.com/gen7ou-561871129
Here's one vs a pretty standard team, Raikou is able to clean up at the end and use Fini as fodder.
http://replay.pokemonshowdown.com/gen7ou-561760557
It isn't the best mon, and it has its flaws, but its strengths are definitely that of a viable Pokemon to me.