The first couple of levels (read: half of it) are doable if you can reliably deal with priests and wizards while distracted by a great number of base orcs and the occasional troll and ogre. The latter half, though, can start throwing sorcerers (which can ignite temporary chain summons and spew demons in ridiculous amounts anyway, and blow you bits even without that), knights, which can render all those base orcs potentially dangerous, high priests, which are base priests on steroids with fire resistance and demon summoning, and warlords, which are just plain beasts. Those last ones are a bit rare, but the potentiality is there and dangerous.
Berserking, especially, is terribly risky in the mines, as the levels tend to be small and the likelihood of something dangerous coming upon you while slowed (or worse, unconscious) is quite high. The standard advice, though, is to do lair (but not its branches) first. (Bolded for emphasis.) This is one of the harder things to pick up, that the orc mines are (sometimes considerably) more dangerous than the lair is, despite the mines (usually, at least) appearing earlier in the dungeon.
Interestingly, your berserker's best bet is probably to just clear the main dungeon as far as it can, for now; both open spaces (Lair) and cramped ones (Mines) are poor situations for them, who generally want to be able to berserk against single or small groups in controlled situations (Read: corridors) rather than against great numbers (mines) or in poorly controllable situations (lair).
The counter to that statement is, if you've got a flaming weapon or are specializing in maces or spears, you'll probably do alright against hydras and the majority of the lair shouldn't be too much trouble for you (at least until you're hitting death yaks) so long as you go easy on the berserk. Berserk's a panic button and should generally be treated as such; any situation in which you have to use it is a sub-optimal one. It's a powerful, but incredibly dangerous, tool.
Re: DJ. The cloud does make most of the mines trivial, as you say. Exceptions being wargs, some uniques, and some of the nastier high HD stuff which the cloud is unreliable against.