Are you sure? What percentage would that debuff be? Because i noted that when i was summoned, but tought it was normal, like DS1 and 2

Its exactly like that. I just played with friend and i was so weak (actually WEAKER than when i was at that location alone...much weaker) i was nearly worthles...

From what (admittedly little) testing I have done, it scales you down, but not very hard. And your weapon keeps its upgrade level, so my +9 crystal sword is still one-shotting low leveled enemies.
My health seems to be about 80%ish what it should be, and armor doesnt scale downwards either, so it feels like being hit with a wet noodle.

Yeah it has always been like this since DS 3 / Bloodborne etc. with the password. The player ranger level isn't taken into consideration for summoning a friend / someone who uses the same password than you. If it's like DS 3 then if the one summoned is a really higher level with higher weapon level then he will be scaled based on the summoner's level if I remember well. That's what Twinkies do they get end game weapons, the barely don't upgrade them, keep a low level and invade new comers who started a new game.

why in the hell wouldn't we twink when it's pretty much guaranteed 3v1 everytime?

Yes

Yes.

I also managed to play with my overleveled "Generally Solo" character with people who just started the game. You get nerfed to hell and back, and only get half of the flasks in your possession. That way it's still fun even if you should virtually steamroll everything in sight.
VaatiVidya group, SEEKERS, seems to work decently for coop too, but it's rare to find somebody with that pass in the multiplayer slot instead of the groups.

It does have a limit, +25 for normal weapons and +10 for special weapons

...what?

I can understand you wanting to help others, but this is where balance comes in. What were you expecting from this game, exactly? When the remake for Demon���s Souls came out, there was little to no downscaling and even an implemented password system. From knows what they���re doing.

If you get summoned by a low level and you two shot their world's bosses, you're not helping, you're ruining their experience.

Weapon Upgrades do play into it. I think FROM purposefully stuffs a +8 weapon down your throat to disable you from invading noobs in the early areas of the game like Limgrave or Stormveil Castle.

I've been wondering why the people i've invaded in stormveil at level 34+4 have **** like rivers of blood and sword of night and flame
i can't believe i spent 6 hours leveling this character, solo'd morgott at 34 only to be ****ed because i killed edgar.

For now I'm keeping it like ds3 with special weapons equalling 2x level of a regular weapon. I invade lvl 40 +8/+4 somber.