Because on-the-job doctoral studies need to give consideration to both work and study, time and energy will be more limited than full-time graduate students, and the unit may not give enough support and understanding, which will bring difficulties and pressure to writing papers.
However, some units will give some support and help, such as arranging time and providing materials, and some policies will also give some preferential treatment and care to on-the-job doctoral students. Therefore, the specific situation depends on the policy and support of the unit.