IMHO the reason this map is hard is because the player is often fighting the environment and monsters at the same time. There are slime pits and traps, hardly any environmental suits, flaky ladders and a general lack of floor space (tech clutter) to fight enemies.
I personally found the ladders frustrating, Quake does not support them and the current solutions (constantly jumping up a series of steps) is problematic. Combine the ladder problem with having to fight enemies at the same time and you have a recipe for frustration. This map could of easily had platforms to exit the slime areas and it would have gotten the player back to fighting the enemies not the environment.
As a rule the more things a player has to deal with the greater the difficulty. Monsters + Ammo/health management + environmental hazards + unforgiving traps + cramped locations = frustration.
If you want a good example of a map with excessive environmental hazards + hard monster layout play Conflagrant Rodent
. This map has plenty of lava, small warped walkways to fall off, lots of heavy range monsters and punishing skill levels.
I personally think the flow of the final arena is wrong, it is setup as a corridor with no loop routes. This is the main reason why DM maps make bad SP maps, the flow is wrong. Typically a DM map is about forcing encounters, players are pushed into focal points to fight quickly, kill players and then exit looking for more players. The flow of DM is the connectivity between arenas not the internal flow around an arena.
The main route though the final room is a central walkway that leads the player to exit the room, you are naturally pushed from one side of the room to the other. The upper area is not used because the flow is linear, move up ramp, jump off ledge. There should of been more ways up to the higher ground, lifts at either end etc.
The monsters in the final room mostly spawn on top of crates and the room then turns into a series of turret encounters. There is so much tech clutter around that it is easier to exit the room and snipe from the doorways than try to navigate the room.