We had a ljet III that outlasted ljet 4, ljet 5, and ljet 4000. Ljet 3 was the last with the HP print engine, afterwards they used Canon print engines.
The network interface was brittle, even a nmap would hang the printer. So we firewalled it off and used CUPS to handle postscript -> PCL. Sending only PCL to the printer (postscript memory and CPU is unbounded) made them faster and MUCH more reliable.
Different handling of duplex, monitoring ink levels, file formats (PS? EPS? PNG? PCL? Which versions? Etc).
Issues with ink that expire by date, reduced functionality with 3rd party inks, not being able to print black even when only yellow is out of ink.
Different postscript versions and the nature of a language where CPU and memory use is unbounded means you get a nightmare of which files can print to which printers.
Most of our printer nightmares, at least the software issues, ended when we handled postscript -> PCL (a raster based format) on the server side.