You're not wrong, but the problem with that is now you're just making stuff up to make life difficult for your players, and I think that kinda defeats the purpose of the game. Challenging the players in Deathwatch is very hard. Last time we played we fought a room full of Kanz and Deffdreadz, and hardly broke a sweat. Then we went and fought a small army of Mega Armoured Nobz lead by a Warboss riding a Squiggoth. Came out of that no issue problem.
The only two times where we've had things go badly for us is when we had a round from hell (my gun jammed, followed by two major Perils resulting in one Psyker vanishing whilst the other one summoned a Nurglesque Daemon Prince that stomped him in HTH combat), or when one of the players (usually the same player) insists on taking something huge on with just his knife (he took on a Hammerhead, one with my Meltabombs already strapped to it, and the bombs blew him to pieces, or when he took on three crisis suits with his knife... and got blown to pieces).
And it's the same against other Marines as well. We played through the adventure from the last DW book - Emperor's Chosen or whatever it was, the one with the really complicated heroic legacies. Came up against that bunker with the Chaos Marines led by the Champion in Terminator armour. No problem at all. 4 Marines, out numbered by Chaos Marines and a guy in Termy Armour, and they walked over them. It was only slightly more ridiculous than the swam of Raveners that they took on before that that crumbled before the Kill-Team because they couldn't Dodge the Marine attacks, couldn't hit the Marines 'cause they kept Dodging, or couldn't do any damage 'cause they didn't hit hard enough (Scything Talons are a useless weapon).
I even remember a session where my Black Templar Sword Brother took on and defeated a Keeper of Secrets 1-on-1 because I had Counter-Attack, could Parry on over 100, and wanted the Keeper to attack me because every time it did I did more damage to it because you can't Dodge on your own turn, meaning the Keeper had no way to stop my Counter-Attack.
Some of it comes down to needing fail conditions for non-shooting Tests. Right now shooting fails on a 96+ (or worse for some high-ROF weapons). Melee combat has no such problem. Neither does Dodge/Parry. If you dropped your weapon/fell over/took half-damage from a 96+ melee swing/Dodge/Parry, then there'd be some extra challenge in there. Right now a natural 100 on a melee attack means nothing other than
"I missed"
. It should mean more than that.
BYE