It has been my experience that repairs and failures have less to do with tube or solid state technology and more to do with parts and design choices. If a manufacturer like Marshall uses pcb board mounted jacks or plastic jacks, it really doesn't matter what type of amp it is because those designs will fail. I prefer tube amps overall and find heat related issues like cold or cracked solder joints occur more than solid state (especially EL84 amps IMO). With solid state, I find more problems with MOSFETs or transistors. Bottom line, I don't think either technology is more reliable, just different types of problems. Well designed amps of either should be reliable. Digital amps are a different problem because there are processors that are difficult to replace and a lot use surface mount technology. I consider these less repairable but I don't know if they are less reliable.