Why is Illustrator 8 format more reliable?

Good files are reliabe in just about any format. Illustrator 8 is deemed more reliable because it does not support many of the more questionable features and efects of later version. Things like Gradient Mesh, Transparency and the like are novel features that cause headaches to output depts. They are not so much bad on their own, but they often require a specific set of circumstances and user knowledge to output correctly. Since at that time {and some say still} the main output destination is Quark 4.11, anything that did not output correctly in Quark 4.11 was deemed unreliable.

Not really fair, when Adobe was trying to push the boundries of DTP technologies, but for something to become a new reliable standard, it must first work reliably and easier that its predecessor.

Any function that looks one way in a live AI file and saves differently as an eps, especially without an intrusive warning, is unreliable. The print industry, rather than learn the ins & outs of these features developed a short answer of "Don't use them, and save as Illy8.

It is not a correct answer, or an enlightend one... but it is the answer you are likey to get.

This unfortunate tenant is at the crux of most upgrade hesitation...