Well, Thousand Sons are basically undead since they're spirits bound in armour, but that's a special case
Most Chaos Marines aren't sorcerers, although they may have some marks or favours of gods, most are just highly experienced Marines and warriors with a taint. That means that one-for-one they should be marginally better than loyal Marines, and that's before you even get to the champions with the real power of the gods behind them.
As for the Guard, the worship the Emperor. If the Emperor is gone then there could easily be a huge loss of morale, which is rather unfortunate when you're relying on morale to keep your cannon fodder being cannon fodder!
From what I've picked up of warp travel/the Astronomican, it wouldn't just be difficult to travel without it but nigh-on impossible. The distances are too huge to cover by conventional methods, and without the Astronomican then there is no judge of how long you have been in the Warp. Because of the shifting time you could be in there for a year and travel 10 light years, emerge, check your location/let the Navigator rest, go back in for half an hour and be another 10 light years on. If you didn't have a marker to judge distance by then travel would be nearly impossible to do right and you'd approach a similar situation to the Dark Age of Humanity.
In the more recent books then Marines may tear through daemons, but a Marine is only a genetically engineered human. A daemon, on the other hand, is a living entity made of the matter of the Warp. Most Marines wouldn't have the weaponry to kill it, only banish it temporarily, as the weaponry is generally limited. Also, I think the earlier stories of the Horus Heresy didn't have quite such weak daemons, but then again GW weren't trying to flog masses of books back then.