What are disadvantages of preventive maintenance?

The downside of such a strategy is that it assumes a mean-time constant operation , which implies a constant failure rate , when in reality equipment is aging ( wear phenomena ) this results in a decrease in the average uptime

disadvantages  happens when its schedules in wrong time , for example if you do it too early thats mean you are losing money and time for nothing as for example changing engine oil before it  reach its limit ,  and if you do it late after failure its also waste  for example if you do engine oil change and filter  after  you  already  got engine heat up and warning lights on dashboard then it is a waste too cause damage already happened so timing is important 

Production loss only

Some times, material, filters or oil is changed but quality is good.

avoid any kind of failure that prevents the prtoductivite

Loss in production, Schedules, Time & Manpower

The most notable disadvantage of preventative maintenance is the down time of the equipment. I couldn't begin to explain how many times I had to hear "are you done yet, sir?" while I was carrying out periodic preventative measures for warehouse trucks. Especially for FMCG and Logistics companies that cannot handle any downtime for one of their trucks. 

This constant rushing causes the person carrying out the measures to make mistakes and/or neglect some important aspects of the test itself, rendering the whole thing a useless waste of money and time.

• Does not eliminate catastrophic failures
• More labor intensive.
• Includes some invasive activities that have the potential to cause incidental damage to components.

If preventive maintenance according to schedule whatever the costs to maintain the equipment life

I dont think so it has disadvantages, my self i change my car oil as planned to prevent breakdowns. As well preventive maintenance has to be planned and to save time , money and efforts.