Mixed armor isn't bad per se, in fact like you said I generally mix armor myself. However, the question with mixes of armor is whether they are complementary and what you are trying to achieve with the mix. Mixing Carrion and Berserker is not complementary, instead the two sets pull in completely different directions in terms of the rotations you should use them with.
In general, I dislike going pure damage because unless you are very good and very lucky you will spend a lot of time eating dirt and thus doing negative dps (you cost the group dps in reviving you while doing none yourself.) Mixing Valkyrie & Knight or Soldier & Berserker gear is generally the easiest way to achieve a good blend of defense with direct damage offense. Other pieces, like Cleric, can be included in place of some of them too without really detracting from the build.
Wherever you decide to draw the line between survivability and offense once you do draw that line you want to maximise the damage component as much as possible. That means deciding which attacks and attunements to focus on and gearing to make those as strong as possible. Obviously you should still be swapping attunements but when it comes down to it do you plan on stacking Eruption or Lava Font? If Eruption, you want as much Condition Damage as possible. If Lava Font, as much direct damage as possible (and we do have more attacks that deal direct damage so building for that is a very valid choice.) If you are focusing on direct damage attacks then your bleeds and burns are largely incidental bonuses and you get much more bang for your buck if you just rely on your points in Earth & Might stacks to boost them - investing instead every dot you can in precision/crit damage. Once you've started to go the Direct damage route every bit of extra investment amplifies the early investment and makes it even better - a point of Condition Damage is a point that isn't making your awesome crits occur more often.
If you don't have much to allocate offensively or focus primarily on utility instead of damage then Condition is a better investment as with half the stat budget you achieve much more (I'd argue that you have to heavily stack both Pre and CD to get excellent results with direct damage.) 25% Crit Chance and 25% Crit Damage is very mediocre while just 1,000 in Condition can still put out really solid damage while having a lot of freedom to use low damage abilities between the big bleeds (especially once you throw Might stacks on top, my Eruptions do about 9k each in a massive AOE.) Unfortunately Condition builds have to stack Power (I wish I could get Vit/Tou/Cond gear!) which is somewhat wasted, but at least means that all my normal attacks don't hit like complete noodles even though they almost never crit.
Basically go either Direct Damage or Condition Damage and then mix some degree of resilience in with that. <ixing the two types will give just you a lot of mediocre attacks instead of giving you a handful of really awesome ones to use between utilities (and DD gets great auto attacks too, while Staff Condition doesn't and instead relies upon Weakness/Healing on its auto attacks to make them useful - so go DD unless you like playing Support.)