It strikes me that two principles commonly used in textual criticism can actually cancel each other out.
1. Charity to the Author: Other things being equal, it's generally better to be charitable to the author when we can do so. If we find two readings in manuscripts, where one makes a lot more sense for someone to have written than the other, then we might favor the one that we might more easily expect someone to have written and try to find some other explanation for the divergent reading.
2. Hardest Reading: Other things being equal, textual critics generally prefer a reading that is less likely to be what you'd expect to find, because copyists can see something and auto-correct it as they are copying. If they find something they consider to be grammatically, semantically, historically, or theologically incorrect, they might fix it. So the harder reading is often taken to be more likely, because we can explain why the manuscripts with the easier reading exist, when it's much harder sometimes to explain why the manuscripts with the harder reading would have arisen from the easier reading if that had been original.
These principles do seem to me to go in opposite directions, since charity seems to support the easier rather than the harder reading. I haven't done a lot of textual criticism myself, but I've read plenty of instances of authors writing about particular cases, and I have to wonder if sometimes people might choose one or the other of these in order to justify the reading they prefer, since charity supports the easier reading.
Does this make textual criticism completely subjective, at least in cases where these two principles are the only relevant ones that apply? Not really. I tried to state the principles carefully enough to hint at how the potential conflict can sometimes be resolved. Charity leads us to look for an alternative explanation for the harder reading, one not having to do with authorial intention, since it favors easier readings we'd actually expect someone to like. The Hardest Reading principle gives us an alternative explanation for how easier readings could arise, but we still need to make some sense of why someone would have authored the hardest reading, or else we might wonder if it's not original, provided that we do have an account of how the hardest reading could arise. Sometimes a slight different in how one letter is written can provide that explanation. Sometimes the harder reading still makes plenty of sense but requires some more careful explaining to see how it fits with the rest of the passage or some other passage. But in many cases there will be a reason to prefer the harder reading or the more charitable reading because of what we might say about the alternative reading.
I do have to wonder, though, about cases where the harder reading makes absolutely no sense, and the more charitable reading can easily be explained by being copied wrongly from the harder reading. There are hard cases in textual criticism because these principles do run counter to each other.