Note: This post is about NPCbots, not playerbots.
To obtain NPCbots, you have to have at least VIP version and then type ingame:
.npcbot helper
in worldserver.conf you can change some settings regarding NPCbots.
I personally find a damage multiplier around 2.1 for melee and 1.9 for magic/ranged the best experience for me at the moment.
Regarding the NPCbots themselves:
I did Ragefire Chasm, Blackrock Depths and The Headless horseman without any difficulties, with out-of-the-box settings.
And in regular pve they perform ok.
I had issues though with
Vortex Pinnacle
when one of the playerbots did 'a spell' on a Star the server just hanged: didnt respond to any server commands like 'account onlinelist' or 'server exit', nor did any mob or npc perform any action besides simply standing there.
Apologies for omitting any specifics, for I couldnt determine them at the moment.
Wailing Caverns
the final boss, where the npc's don't do enough aggro and Narilex's disciple dies
did it once though, with a rogue, and might be my own noobness at play here

Update: "damage multiplier tweak in worldserver.conf" and a better player

should resolve this 'issue'
Deepmines
the second boss, Helix gearbreaker, doesnt do what its supposed to do.
Either doesnt die and keeps at 1HP
Or picks the player up and starts running, through the wall after which you fall to your death
not entirely sure here if its the playerbots or the dungeon itself tho
Update: this is a bug in the dungeon itself and known
Lost City of the Tol'vir
Third boss,
High Prophet Barim , in the second stage, all the playerbots lose health very fast.
I ran this boss maybe 9 times with different tactics, once or twice I reached the third phase with 1 bot left and simply couldnt finish.
played paladin tank myself.
Update: with v13.1 Gold I have completed this dungeon on Heroic setting once. (I was tank, with 4 NPCbots)
Edit:
replaced playerbots with NPCbots
added updates