Two different players with different stories:
Farabee was an up and comer, he was on his way to being a solid 2LW, maybe even a 1LW, then he hurt his neck at 22. Since then he's been unable to add strength, he's a slightly below average skater, still a good scorer (on the first line in Calgary with Huberdeau and Kadri) but I think medicals scared teams, no one was willing to take a 24 year old player without retention, and I doubt anyone was offering a 1st. Briere showed in the Provorov and Walker trades that he knows how to extract value, but sometimes value isn't there. This was basically a salary dump, Kuz will be gone this summer.
Frost has been an underachiever since he turned pro. Not for lack of trying, he worked hard to improve his defense. but after a solid season two years ago, he regressed - despite having the best wings (TK, Tippett, Michkov, Atkinson the last two years). Now he's the 3C in Calgary behind 35 year old Backlund, and playing much worse with less talented wings.
The real issue is both players are "soft," in that they're not very physical and don't suit the aggressive forechecking, go to the dirty areas style that Trots has implemented. While Torts will make allowances for highly skilled offensive players, you have to produce.
The trade was more about cleaning out players who didn't fit Briere's vision going forward. Fanbois like to blame Torts, but I think Briere is as tough when it comes wanting players with "grit," he was a smaller player in a far more physical era so he doesn't buy excuses. Play "big" or play elsewhere.