Death exits are just plain frustration and trolling you at the same time - I shall never ever utilize them, that's for sure. There'll be many levels with "TakeInventory", but no death exits whatsoever. It's good for a quick laugh and exploiting a glitch if you're really low on health, but generally, using a game bug as a feature or a must-know solution for levels is greatly frowned upon, much like hiding a necessary key in a super-well-hidden secret area.
The weapon-monster balance can depend on a lot of things. In an average level with no special conditions, death-traps and other pleasantries, I'd say a pistol and a shotty is in par with Zombiemen, Shotgun guys, Imps, and Heavy Chaingun Dudes. Chaingun is in par with Cacodemons, Hell Knights, Pain Elementals and Lost souls. Rocket Launcher is a preferred choice against swarms of Cacodemons, Barons of Hell and Revenants. Plasmagun is essentially a slightly stronger Chaingun, so I guess it can be used against those beasties. As for the BFG, it does well against swarms of Barons, Mancubi, Arch-viles, and of course, the big bads, such as Spider Masterminds and Cybies.
Of course, this highly depends on the available ammo and the environment. If there is only 2 Energy Cells, you will probably conserve the two BFG shots, and use a chaingun or a shotgun instead. If the area you're in is cramped, and has short tunnels, using a Rocket Launcher is a very bad idea, even if you're up against Barons of Hell. If you're totally low on ammo, a Berserker Pack or the Chainsaw can be HUGE help. You must also take nearby traps, explosive barrels and monster infightings into account. If you're skillful enough, you can massacre 80% of the monsters on the level
MAP18: The Courtyard without firing a single shot
that map is friggin' easy even with a pistol-start. I didn't believe it until I actually tried it. I lost 42% health (a Revenant hit me from up above), but at the end, the only monster I had to finish off was a terribly weakened Hell Knight.
It is very possible to make a stage hard to complete even without placing a simple enemy. A huge variety of death-traps, lethal pitfalls, some sort of slalom-course in zero gravity, adding airstrikes a la Half-life, adding Miragia effect platforms, creating series of tricky jumps, creating a few challenging (but not impossible) puzzles, or adding a very strict time limit. I think it is a very nice addition of a game when the focus shifts from simple open door-grab ammo-kill monster routine