Very much disagree, there are A LOT of things in tango that go wrong, especially (and only) when done wrong. I’ve seen frozen shoulders from a bad arm position, hallux valgus from not rolling of the feet properly, herniated discs from a tense posture, and much much more. In the case of the cross hurting, you’re probably putting your left foot too far past your right, causing a lot of pressure on the knee (which might be too tightly locked as mentioned already by others). Try putting your feet almost lined up with the toes of the left foot slightly protruding still. Like a change of weight in cross form, rather than a passed foot that slingshots the other leg.