In a nut shell.. OEM harnesses are perfect easy to install and have all the right connectors and plugs, Universal harnesses do not.

For GM and some AMC/Jeep owners, they will find that universal harnesses actually have the plugs and connectors built into the harness for thier column. But not always.

For Ford and Mopar owners, there are some companies making a few bits and pieces of OEM harnesses but not many. This was due to the fact the plugs are not available anymore and the cost to tool them can be expensive.

For Import owners and other make pre-fuel injection cars. Universal harnesses are not only the best option, in many case the only option since OEM does not exist for thier make or model.

Simply, Universal harnesses are universal and by thier nature can work in any car, the only real problem comes with model specific issues. Things like the Opel GT headlight which have a microswitch to turn the lights off in the down position are a trick to rewire with a universal harness, but not impossible. The universal harness gives you the basics and you have to be invovative from there.

On the positive side, universal harnesses on eBay can cost less than $150. When compared to the cost of a OEM harness (Usually $500 or more). A little ingeniuty can save you hundreds of dollars.