Cannon vs. demolition.

Twenty levels less. That's what... 2k?

Vipers don't gain the same amount of HP as warriors per level, and they don't have as many HP bonuses from 1st and 2nd job advancements. Keep in mind that Pirate base HP gain is the same as archers and thieves (as demonstrated by Captains having roughly the same amount of HP).

Their Improving Max HP increase also only adds 30 HP per point rather than 40 like warriors.

So they're gaining 20-24 HP per level until level 30, when they gradually move up to 50-54 HP. They don't get the HP bonus warriors gain, and 50-54 is quite a bit lower than 62-66.

I don't have exact numbers for the kind of HP Vipers will have, but I expect their survivability to be on the level, or slightly less than Shadowers. They'll need HB, or some HP washing to survive HT before end-game levels.

Edit: Pulled a random screenshot of a Buccaneer.

 Spoiler

Let's call it 3850 HP at 82. That's pretty high, a lot higher than my BM at 122. With a gain of 50-54 (avg 52) a level, they'll hit 8k hp at level 162. With some HP washing, that's probably 13x-14xish. It's roughly on the same level as Shadowers. My main point is, they definitely aren't as durable as some of you are making them out to be. They are not NLs with warrior HP. Heroes hit 8k HP around 120 with absolutely no washing (I think). It's well over a 2k HP difference by 16x.

And what's with the F3 at 2k HP? I only have 3.3k on my BM with 200 HP from equips. 2k is a lot.