I specifically went to YT and looked for pvp tier (done about 3 months ago) and one had the arsenal as C/B-tier (arena/wz) and the other as A-tier, and both used the S-D scale, a reliable source of information for someone starting pvp lol. DO NOT USE YT AS A SOURCE OF INFORMATION.
The facts are that there is no shortage of people playing as arsenal/gunners in PvP, what's more, they regularly end matches with a large number of kills (3-5 in arenas and 30+ in wz). It's true that they have the worst damage, but the statement that you can't kill anything with them this is quite an exaggeration, with the current state of pvp you can kill anything you want with them.