The lack of access to your deck (tutor cards) and deck thinning is the reason these cards have become more or less staples in the game.
There are other thinning tools and tutors in the game though. The problem is other thinning tools cannot compete with Witchers for a number of reasons and most tutor cards are neutral.
A good example is comparing Witchers to, say, Mahakam Volunteers or Impera Brigade. The former is 21p for 12 pts on the board, 2 deck thinning and comes as a 3 card package. So in comparison...
1. 12 pts vs 6 pts
2. 12p for 6 pts vs 21p for 12 pts, slightly less efficient
3. 2 thinning vs 1 thinning
4. 3 cards vs 2 cards, so higher chance of finding at least 1
If you compare Brigades or Volunteers to, say, WH Riders or BSC it's a bit more favorable for Riders/BSC because their potentially 8 pts for 12 provisions and 1 thinning. The WH rider condition is easier to satisfy by going first in a round or playing a big card. The BSC can get more point value by copying them with Scouts. Notice people play these cards. Volunteers and Brigades see considerably less play.
You can probably put Skirmishers with Skalds/Birna in there too, although Discard is slightly different. Shieldmaidens would be a more accurate comparison but those are typically 7 pts for 12p and can be removed.
Witchers can also pull Roach, giving a 16 pt play. This is pretty valuable on red coin since you can stay 15 pts behind and still over-take in 1 card play. It's valuable on blue coin because you can lead tempo to avoid getting behind too far. It's valuable on both because you can thin by 2-3 in one card play. Despite the over-provisioning they're still incredibly valuable. The only huge drawback compared to other thin tools is the slightly less efficient provision cost and the mulligan headache. I wonder why people play Witchers.....
The neutral tutors and thinning tools face similar issues. They have the potential for significant drawbacks (Quax, Last Wish, Iris's Companions, Stregobor, potentially Villem), can be very expensive (Dandelion: Poet, Avallac'h: Sage), come as a single card, can be removed (Ciri: Dash, Summoning Circle) and/or only pull one card.
Basically, trio is too strong unless there are other available trio's and almost nothing else can provide the advantages of Witchers. Personally, I'd cut out the trio concept altogether. Split Witchers into 3 separate cards. Add in some faction tutors (2ish) and prop up the under-achieving faction thinning cards. See where that goes. If needed add slightly more faction tutors/thin tools.
