The spine is meant to move into flexion so don’t think it’s inherently wrong to perform this movement. I think when performed correctly, it’s a great tool to teach segmental control and mobility into spinal flexion as well as lengthen and strengthen the posterior chain. I don’t think it’s necessarily an exercise you want to load for maximal loads in the same way you would for exercise such as the DL, but I think under light to moderate loads the Jefferson curl can be of benefit. As with any loaded movement, control is key. If your just passively letting your body fall into spinal flexion I don’t think your getting much benefit and may be more likely to aggravate something, but with good control, I think it can help strengthen the spine into flexion and improves ROM. If someone wanted to incorporate them into their routine I would just recommend starting light (bodyweight) and then slowly add weight as they built segmental control. I would obviously stop if there was pain or discomfort as well. Loaded stretching in general, like the Jefferson curl, are great for building strength at end range as well as improve ROM. Again, the key here as long as these exercises are done with appropriate load and with good control.