As aylmeow said, the big difference between 3 turn farming with waver/merlin to np farming with skadi is efficiency. If you want to 3 turn consistently with the present meta you need at least 2-3 DPS servants to take off the farming CE and put on starting NP, NP strength/buster+. This significantly hampers your farming efficiency, to the point that there’s some debate as to whether 3 turning is more efficient than 7-8 turning with 6 farming CEs. The difference once skadi drops is that she can amplify the strength of one, preferably quick card servant, like Lancelot, such that only one servant has to put on a non farming CE, and they can loop their NP for all waves. This might not sound like a big difference, but the resources really do add up. Another aspect with the current meta is that depending on your lineup, farming might not be consistently reliable, and some of us have to switch teams for different farming nodes. It’s not so easy to farm on autopilot. It takes some effort on each run. It’s better to have a reliable NP loop with Skadi and just effortlessly breeze through runs, maximizing your efficiency. She really is a game changing servant for farming.
Now, is she good for challenge quests? Yes, for some. Consider a CQ with 6 bosses and a CE that amplifies the strength of a single servant by 200%. You build up your NP once, then AOE clear the whole first wave of challenge bosses. Normally you have to build up your NP again, slowing you down and letting RNG dictate the success of your run. Skadi would have just enabled your servant to blast through the second wave right away. This is a bit abstract, but it gives you an idea. Some challenge quests offer such difficult parameters that they’re virtually un-stallable for some rosters. For these, it’s better to burst through them immediately, with buff stacking or NP looping, and Skadi is good for the latter. If what you need is a stall team however, Skadi might not be ideally reliable, especially considering evade is inferior to invincibility, she doesn’t offer team wide NP gain, or healing, as Merlin does.