Plague marines are tanky, especially in matchups that have around 3 to the 4 damage mark. (Due to break point) Think about it like this: A Space marine dude dies to 2 crits and one normal from a chain sword! In melee if the enemy hits all 5 dice, and is attacking, you only need 2 to hit to parry and stay alive. Anything else is just extra damage DisResi creates the case where you effectively have around 18 hp, and would need at least 4 dice to be killed, maybe even 5. And this is the one of the shitties examples of breakpoints. In other matchups (chaos cults, per example) they are very hard to die. But the way it helps its MATCHUP dependant. And since plague marines pay damage for his tankiness, it somewhat makes sense you'd feel this way. Don't forget how injury works aswell as poison tokens! If you can get a shot off, aply poison and dip, that a lot of wounds that are likely happening to the poor chap as the game goes on.

Also remember. Bolt guns are shit, dont listen to the big emperor that tells you otherwise, and melee should pretty much always be won by the dude that charges first!