You won't be OHKO'd by any Sucker Punch in the tier though because it's very easy to play around. As a Sigilyph user facing a Sucker Punch user, you can comfortably switch out because you will never have to worry about taking hazard damage. As a Sucker Punch user facing a Sigilyph, you almost always have to click Sucker Punch because if you mispredict, you lose your best Sigilyph check. I think the only exception to this is if the Sigilyph is severely weakened and might use Roost as you use Sucker Punch because you can't afford to allow it to Roost. Predicting is all about risk vs. reward, and the risk is skewed towards the Sucker Punch user most of the time in these scenarios unless they also have another solid check waiting in the wings.
This isn't a fair comparison. Sawk is almost always Choice-locked, which makes it much easier to play around by scouting with something bulky, a la Alomomola last gen which was 2HKO'd by Adamant Close Combat after Stealth Rock, but could come in and pivot out into a Fighting resist. You can also play a game of prediction against it because of its tendency to run Choice items. If it is running Life Orb, which is viable mind you, then it isn't as strong as Choice Band Sawk and is easier to check defensively.
Against Sigilyph, however, you can viably run Life Orb with incredible coverage and the power to hit things on both sides of the defensive spectrum. Sigilyph is far more threatening towards defensive and balanced teams for this reason. Yes, you do need to run answers to Sigilyph on any team. But this is much more difficult for the aforementioned reasons and the reason why I'm leaning more towards support a ban than opposing it.
I'm one of the people who said that balanced teams would be more viable without Sigilyph in the tier, and I stand by that. No, it won't bring in the age of balanced. There will always be Pokemon in this tier that give troubles to balanced teams. But this team archetype will absolutely be much more viable without Sigilyph around. Unlike stall teams, you can't afford to run multiple defensive answers to Sigilyph on balanced teams, which means that you almost always need to run offensive checks to Sigilyph as well. This is where Sigilyph makes the tier more offensive. If you're not running heavily defensive teams, you usually need one or two things that can outspeed Sigilyph and KO it or things with priority that can do the same.
I find that Sigilyph is simply too good at doing what it does. No, it's not going to 6-0 teams on a regular basis, but it doesn't need to do that in order to be too strong for the tier.