Why are straight legged deadlifts not a good idea to do?
As a strength coach, they are one of the best exercises you can do for some posterior chain assistance work. With that being said the biggest problem that I see with this exercise is lowering the bar below the knees and rounding the lumbar….NO NO NO! The back should never be any more forward than with any major lift (deadlift, squat, power clean, snatch), its when the back is taken past this point that the exercise becomes more risk than reward.
If you don’t like Barbell RDLs then do single leg RDLs with no weight or a dumbell or two.
Just caught the “straight leg” part of the question. To me straight leg deadlift is not locked knee deadlift it is slightly bent knee deadlift (RDL = Romanian Deadlift)