Space Marines are tough, and they only get tougher. Rites of battle are basically to Deathwatch what Ascension was to Dark Heresy. No longer playing level 1 tactical marines, now characters can become dreadnaughts as well as Marine Captains. When the game begins, Marines are tough, but must still be careful because some enemies can still throw great firepower at them. However, as they grow and get stronger, and get more equipment and bonuses, at what point do they become a whirling malestrom of destruction that no Xenos horde can overcome? Similar to Dawn of War (especially Retribution), where you start off weak, but eventually become so powerful that you can just walk around the map and destroy anything that gets in your way.
I imagine the following would make a Marine group almost unstoppable
Raising toughness bonus to 10 ( or even 12 if they take all toughness upgrades)
Getting upgraded armour, such as artificier or Terminator armour
One of the characters being a dreadnaught
Having support, such as a landraider or a lance strike
Master Crafted weapons
Bonuses to combat such as Bolter Drill
An Apothecary capable of instantly healing any wounded characters
A librarian able to errect a protective bubble around the party
Chapter upgrades in defence such as the Dark Angels ability
Squad tactics which increase the survivability of the group (go to ground, soak fire)
All of these are multipliers which, put together, can probably create a party which is truly epic. There must be a point at which the GM cannot challenge the party without resorting to unbelieveable measures (such as when the party can demolish the hive tyrant without breaking a sweat). What are some experiences with Deathwatch characters becoming extremely powerful?