Just by the way, I happened to be rewatching Civil War this afternoon. Turns out this simply isn't true. Lang picks up the tunnel and tries to swing it at Rhodey as a weapon, he never once attempts to use it as a shield. He also gives no reaction at all to Iron Man's missiles beyond just waving his hands in front of his face in surprise and panic.
If you look at the scene closely, you can see that Rhodey only hits Lang with his shoulder-mounted minigun. He only unleashes the heavier-duty stuff on the tunnel.
And also, Lang bringing his arms in front of his face shows that he was concerned that the weaponry could seriously hurt him if it hit a vital spot. There would be no reason for him to do that if he was immune to War Machine's weapons.
Tony's technology wasn't effective against Thor. It was just enough to keep him from getting owned completely, even though Thor barely seemed to be trying and viewed Tony more as an annoyance than anything else. All it was capable of doing was knocking Thor off his feet a bunch - and every single time Thor jumped backed to his feet without hesitation, and never a single sign of pain or damage of any kind.
I don't see how this is relevant. Thor is on another level to Valkyrie altogether, given that unlike him (or Hela) she isn't considered a god. We have no reason to think that Valkyrie is any more durable than Sif or the Warriors Three.
And sparring with a skilled fighter is not the same thing as sparring with the Hulk, who can crush you with one finger and isn't exactly known for his fine motor skills and impulse control. If she could be taken out by weapons that didn't even hurt Ant-man, she would have no business in that situation.
Hulk clearly developed a decent amount of self-control on Sakaar-hence his ability to wear clothes, bathe, talk etc. He has far more control over his faculties than ever before and is no longer a savage monster. As a result, its not unreasonable to assume that he could know to pull his punches a bit when sparring.
And anyway, Valkyrie could just use her obedience disk on him if things got out of hand.
You're right there's a lack of evidence, but it's a lack of evidence on both sides. The only actual Asgardian Tony has fought was Thor, and his technology proved basically useless there. On the basis of that there is no reason to believe Tony's armor would fare better against other Asgardians.
Ok, so Tony's weapons were ineffective against the second-strongest Asgardian ever, therefore they would be ineffective against much weaker Asgardians.
Seems like sound logic.
Maybe Valkyrie is so far below Thor's level that he'd have a chance, but if Odin thought her capable of killing one of his children, it doesn't seem likely.
Odin most certainly did NOT think that Valkyrie could kill Hela. He thought that ALL of the Valkyries together MIGHT stand a chance of subduing her. And he was wrong. As Odin usually is.
On her own, there's no reason to believe that Valkyrie would last any longer against Hela than the Warriors Three did.