Many old units work fine. Due to their intended use and high cost, quality components were often used. Common failures are capacitors and mechanical components that just need a lubrication service.
The old Pevicon tube based cameras still work, but often need a 'tune up' to compensate for tube aging. The Lead Selenide based cameras are very reliable with only the vacuum in the detector array capsule to be concerned about. BST FPA based cameras are often still working fine but some suffer failures of Tantalum capacitors. The very early nitrogen cooled scanning cameras will often work fine as well. Some suffer age related issues with the scanning mirror servos. Some high quality end cameras encapsulate the complete mirror block in a vacuum capsule and the vacuum can be lost over time.
Older thermal cameras will often work with just a little TLC. Sadly the pictures they produce can be disappointing to those unaware of how far technology has moved on in terms of resolution and image processing.
Fraser