Gengar bad ?

He is very frail and weak to extrasensory, so he will die fast. The thing is, he does alot of damage in a short time, so he is valuable if you fight in small groups. I wouldn't use it otherwise.

He's listed as glass canon as counter for lugia on the website of Gamepress. This means that he has a high damage per second (DPS), but will not survive for a long period of time (therefore the name "glass", very fragile).

I mainly find glass cannons to be relevant in solo raids, because it is important to win time when you only have 180 seconds on the clock. Other than that, I wouldn't invest in them for tier 5 raids.

A pokemon at level 20 is almost never viable to use in a raid. Especially with revives being less plentiful.

Evolve another one to Sucker Punch + Focus Blast and you can attack most gyms.

I use one with the same moveset in the first spot in Lugia raids, because he can usually launch two shadow balls before being killed by a charge move (definitely against future sight and hydro pump, but not always against sky attack). If he launches two shadow balls, he is doing a lot of damage in his short life. In any spot other than the first, the randomness of Lugia's charge moves might cause him to die before launching his second shadow ball.

Shadow claw Gengar can generally not get off two shadow balls, due to lower energy gain, so this is one of the few matchups where H/SB seems better than SC/SB in actual practice.

Glass cannon is a good option when raiding with few number of trainer
I just use it upfront and follow with tankier but still good counter, so i make 2 battle parties for that

For example, against Lugia my first battle party is 3 gengar, 1 mewto, 2 jolteon
2nd is tyranitar even against hydropump variant.
Lugia is not kyogre level as a hydropumper, so 6 tyranitar still get the job done maintaining damage.

For raiding with many trainer, i am not using glass cannon at all, unless they are many new or back from retirement trainers just in case

What level is it

It works well against Hydro Pump Lugia if you put it onto the first slot of the team because Lugia will take time charging Hydro Pump and you can fire like 4 to 5 shadow balls before the hydro pump hits it and it dies.