All of the sunroofs with only forward/backward movement were very reliable. If the cable/housing becomes a problem it can be expensive. The number of cable problems with these roofs (in all models)for all our 22 years was surpased in the first 2 years of 124 body lift back roofs.
Keep the tracks clean and slightly lubed and don't force it if it refuses.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician