These types of events also award most potent of event units in average. I mean, if not for Saki, even Nagi would be quite a star of free player's team
And, to be fair, only event type that absolutely don't require spending SCs to perfect it is Star Trial, as you either have enough firepower (of needed types) to clear it all, or or you don't. And even then there's an option to burn through your stash to power-level your units. Even the humblest of Unit Farm events could go terribly wrong even for veterans, with target unit refusing to drop and what little drops refusing to proc Skill Up and Cost Reductions, with everything in it being left to the mercy of Random Number Goddess (and we all know how she can be sometimes). So yeah
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