My thoughts on these two issues.
GM Burn Out. This can be a real issue. WFRP 3rd is an easy game to run but a very hard one to master. I have run 70+ sessions of this game and I end most with a list of things I would like to do better. This sort of pressure can leave you feeling flat. Sometimes this is caused by players. You have to remember that it is not the GMs job to entertain the table. It is the GMs job to facilitate the people at the table entertaining each other. If you have players that suck the fun out of roleplaying, even just one at the table, then this will drain the energy out of you. My advice would be to mix it up with different games and different players from time to time.
Combat and Character Death.
Character death is normally a bad idea. Roleplaying is meant to be fun and that fun tends to come from a variety of sources. The fun is to be had by both the GM and the Players so there is a symbiotic relationship going on. Sources of fun include:-
1) table talk and banter
2) Story telling
3) Character development (both in story terms and mechanical terms)
4) World / setting exploration
Source 1 is a given and unlikely to be effected by a character death (although see note below) and 4 is pretty straight forward as well. Sources 2 and 3 can get ruined by character death. Players normally enjoy developing their characters (which is why there are so few games around without an experience point mechanic) but this is not possible if they die on a regular basis. GMs and players want to create stories but this is ruined if the lead characters die at strange moments. Raiders of the Lost Ark would not be as good if Indy died half way through and a random archaeologist finished off the rest of the story.
Character death is good. Without character death you have no risk and a lot of tension, and fun, is lost.
So basically you need to have the threat of death as often as possible but to only actually have it occur occassionaly.
Another point is that you want your players to be upset by a character death. Different players handle it in different ways. I have had some walk out of the game and others just start rolling up a new character. Ideally I would like them to be somewhere inbetween. When I am a player and my character dies I like to take the rest of the session out to work out the background of a new character and a means of introducing them into the game.
I have recently played in a couple of games with GM's who had opposing views on character death and they both failed in different ways. Both where Pathfinder campaigns but the results would be the same in any other game. In game A the GM was keen on killing as many characters as possible. One player (in their first every RPG experience) was going through character every other week and every single singe player was on to their second or third character within a couple of months. The game fell apart. None of the players enjoyed it and the story became non existent (because all of the character created with strong hooks to the main plot died). Players stopped creating new characters and just introduced the new one as "my previous characters twin brother". This was probably the last straw for me as a player because I like to play with characters rather than stat blocks. Game B the GM was against killing characters if at all possible. The story progressed much better and eventually after about 4 months of play the first character, mine, died. No problems, I new enough about the story at this point to create a character with sensible links to the plot and we continued. Then two weeks later my replacement character should have died and the GM ruled that he survived. Similar things then happened with other characters. I.E. the bad guys would attack a character until they were at 1 or 2 hit points and then, for no good reason, switch to less damaged characters. Pretty soon the players lost a bit of respect for the GM. However, GM B kept his game going and most of the players would probably play in his game again (although they might mock him for being a wimp GM until his kills someone), whereas GM A's game fell apart and most of the players would not join one of his games again.
So its all about balance.
Final note on published adventures. The combat readyness of a WFRP party is much more variable than it is in other games so often you wont be able to take the enemies in the published adventures at face value. Examine your party and tweak the encounter to suit.