From what I've read on forums, many of those 'Adventurer Factions' (as I believe the code calls them) are hard-coded to not actually become friendly to you. For sure, the evil armies of undead and such have that feature. So even if you do something or enter a cheat, your rep will drop into the negative again.

The Knightly Orders are the only ones you can sway afaik.

Someone correct me if I'm wrong about the non-plot-evil adventure groups pls.

As far as I remember only Heretics and Snakes are impossible to befriend (on the long term).
Befriending all minor factions (except mentioned above) will boost your economy (peasants will safely travel between your towns and villages, caravans will be safe) and defence capability of your kingdom (they will join battles against common enemies).
On the other hand you will miss a lot of battles, prisoners and loot.
Try it and see what is more fun to you.

Its been years since i have played this mod. Just picking it up again, but I have definatly befrended some of the minor factions, Barclay, Mettenheim, and Singillans used to be pretty easy. I used to alwasy join battles on their side when they were battling other lords armies. it gave me interesting paths of retreat if I ever got overwhelmed

Noldor