What are the disadvantages of OHC?

The highest point-loading (where the oil is most likely to break down) in an engine is the cam/cam follower interface.

The snag with OHC is this is the furthest distance from the oil.

FORD found this out the hard way with the Kent engine which used to wreck camshafts and followers at very low mileages, but didn't show up in pre-production testing.

As a manufacturer, you have to be very careful in applying accelerated-life testing to real-world usage.

Other disadavntages:

Makes engine taller/larger.
Chains are noisy, belts have unpredictable life.
More gubbins to remove for a head-job.
Probably more............

Overhead Valve engines can be made very efficient using modern tecniques. A perfect example is the TVR Cerbrera, this has a Overhaead Valve 4.2 litre engine (single cam)

Despite this apparent stone-age design this car has the following figures:

0-60: 4.0 sec
Top whack: 185mph
Consumption: 19 to 25 mpg

These figures are not too shabby, who needs more cams?

__________________
Paul Gibbons
'93 320CE
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike