Well besides the external differences, there can be differences inside. The more simple ignition units had no spark advance. Later they had a simple two step retarded and advance mode built in. Then more expensive units put in advance curves or multiple steps to control the ignition timing. Some differences would exist as they could program in different advance curves for different engines based on the need or requirements. The cheaper ignition modules tend to have a simple two step timing with retarded for starting and advanced for running. More expeneive units may have a advance curve in them. I think most all of the ignition modules now use microprocessors in them and are programmed at the factoryu based on their contract requirements.

Other differences is in the voltage level needed to run the ignition module. A cheap module may only work a 4.8v with 6.0v really being the maximum voltage allowed or the module will burn out. Better modules support 7.2v to 8.4v and sometimes even 12v voltage levels to power the unit with.

The issue can be obfuscated by manufacturers ordering batches of ignition units for a specific engine and leading one to beleive it is special just for it. RCExl for example slaps on stickers for every brand as needed in the contracts when they make them.

But with our model engines a generic ignition module will likely work just as well as the branded version. The advance curve may not be the same, but you may or may not be able to tell the difference.