My archers use and abuse poisons, but I've never bothered with fear and frenzy. Today I tried brewing some, only to discover that at 100 alchemy, with all the poison boosting perks, the highest level both poisons can affect is... 12?? Surely that can't be right.

Just checked UESP and it says fear and frenzy poisons can go up to level 23. It does not say anything about the level that a user created potion goes up to so I assume you can make yours better than level 12. Sorry I could not be of more help.

Those fear and frenzy poisons are a nice idea,but they got ripped being capped at level 12.
I haven't tried it myself but I heard this somewhere else.

Sounds reasonable to me, things (like pretty much everything else in the game) having a limit. Maybe it could have been made differently, I don't know, but I agree on the intent. Hell, I'd even like to see more of it, like steel weapons not "working" very much at all against i.e. dwarven armor no matter the skill involved. You may be able to beat a bushman with a spear, but maybe not a tank