We've been playing Deathwatch since release (a year or so?) and we have come across so many glaring 'broken' abilities/synergies that we are ready to stop playing. Please understand that this is just our experience and my opinion!
Yes, we could houserule the hell out of it, but at this point...it seems more broken than working.
Someone might say, "It's a roleplaying game, don't break the system". This is not the point. A game system should be able to 'withstand punishment', otherwise it's a poorly designed game. I'm not talking about using obscure rules and supplements and houserules, I'm talking about abilities in the core rulebook that are poorly thought out. Perhaps a larger criticism is that the game itself either needs major work or is otherwise 'broken'. Some of this comes down to powergaming players (please read what I just said!).
Problem 1) The core mechanic of Deathwatch is a player is 'defence rich, hitpoint poor'. I'll use an average character:
Armour: 10/8, Toughness: 8-10, dodge/parry, possibility of a storm shield (negates 55%!! of attacks); later possibility of iron halo etc, which doesn't even overload. Squadmode ability which lets marines share reactions (forget the name). This means that if a Master level creature (say, a demon prince) is attacking the melee character (let's give him 60+15 from storm shield= 75 WS), he can parry pretty much all the attacks. However, IF the demon prince gets a lucky hit or two in, the character is DEAD. (2d10+25+felling, Pen.6 and adds +4 to crit damage). This means that the character takes 37 average (2d10--> 12, +25) , gets 2-4 armour, and 4 toughness (felling, remember) from an average hit point pool of approx. 22. This puts him on critical 3, add 4 for 'killing blow', critical 7 (doesn't *quite* kill you) in one hit. Now, here's the problem I have and others might not. It's *very* hard to balance fights around a system like this, since it's based thematically on the tabletop game- the 'all or nothing approach'. I find players either emerge unscathed or there are horrific casualties. This only gets *worse* at high levels when they get better stats, gear and stuff and fight harder foes.
Problem 2) Troubling abilities/items
When this game first came out, there was a big problem with heavy bolters- they were OP. However, next to psychic powers...the fact that Librarian's abilities cannot be resisted (seriously...our Librarian rolls at 130+ for his powers without risk of Perils of the Warp) and the sheer damage/utility with no cost, makes them overpowered in the true sense (outshines other party members- don't care if they melt hordes!).
The fact that everyone can take a jetpack and storm shield (eg) and render most fights trivial. (You could *arbitrarily* say they can't have them, but I'm talking about the rules, not someone elses' game here.)
Hordes are trivial except for specific circumstances. (devastator marine who is a dreadnought can usually do about 70 magnitue per round)