I find that this topic can be easily misunderstood. This is my opinion.
If a person is in an abusive relationship whether it is physically, verbally, manipulative etc...it does not mean that you stay in that relationship because you are told to love unconditionally even If you are treated badly, understanding what unconditionally means its important.
For me it means you do not take revenge on the person concerned and on yourself. You accept how this person is (loving him as a human being), but for your own safety whether it is physical or mental you need to distance yourself. You do not take the actions personally, but you understand that the person is acting this way because they have trauma/s that they never dealt with. Having said this the pain caused is still felt whether physically or mentally.
However, to heal yourself from this pain, you have try to see it from the perspective that I mentioned above, that pain somehow will start to diminish slowly. And you love the person as a human being (unconditionally).
Then there is the narcissistic side to this as well. A narcist will treat another person badly and will take a person for granted or use the term unconditional love to their advantage. Because they will use manipulation and make that person think that even if they are treating them badly, they should stay because this is what unconditional love is. Unconditional Love is loving a person and
seeing him as a human being just like you, no matter what they do, with all their faults. Because they have unresolved issues with themselves.
Unconditional love does not mean that you have to suffer any kind of abuse.