1. How many times out of a 100 games have you played against mega gengar? Approx number is fine

I'd say 20-30 times, it's a top tier pokemon for sure !

2. How many times in 100 have you used gengar?

About 10 times, i wasnt using it much in SM but i start in USUM.

3. I met it a lot at the end of SM, like 20 times / 100, but it disappeared with USUM coming in. Maybe there's still 1-2 remaining.
4. 0, i never use it.
5. I'd say 0-1 / 100, i barely ever see it.
6. 0, i never use it as well.

7. Out of the shadow tag users, which is more reliable to ladder with?

I like mega gengar a lot, because it's not deadweight even against offense, since it's really fast and hits hard immediately. Gothitelle is only good (but terribly good !) against bulkier teams, and wobbuffet is not common enough for me to have an opinion on this.

8. Explain your answer to 7 briefly

Done in 7.

9. On a ladder basis, do you currently feel any shadow tag user (or all, or two) give players an unfair advantage?

Gothitelle is really unfair because it only pp wastes your pokemon and there's nothing you can do about it. It's extremely boring and frustrating. Once you're pp stalled, anything can setup for free on you, and when you know how important 1 turn can be in Ubers, you understand why it's broken.
Mega Gengar is obviously really good, but i dont feel like it's broken because it's easy enough to revenge kill (mediocre bulk, any scarfer can at least 2hko it), and it can rarely come in for free on a pokemon, it has to be played well to work at best. It's not brainless as Gothitelle is.
Wobbuffet is good because it can trap most of the pokémons that dont OHKO it, but same goes for mega gengar, it kills a pokemon or gives a free turn (encore is really good for that). I dont feel like it's broken because it's extremely pressured by HO, which is dominant atm.

10. How fun is laddering with shadow tag? Do you estimate it being more enjoyable without it?

Laddering without Gothitelle would definitely be better. Mega Gengar should stay in my opinion, because that's a great countermeasure to stall/balance and i dont think it's an unfair one (i'm a balance player mainly, no personnal opinion in that). Wobbuffet can be annoying, but overall i feel like it's "balanced" since it lacks recovery and can do his job once at most.