String manufacturers/builders all use the same basic materials, dyneema or dyneema/vectran blend. Some string manufacturers/builders may claim to use a "proprietary material" or "proprietary method", usually meaning a custom blend of dyneema/vectran or a special sized serving or specialized equipment.
But since the same basic materials are being used regardless of the maker, you will usually get a good string. The difference between a good string and a great string is in the workmanship. One of the most common problems in a string is usually poor serving quality. These string manufacturers/builders use high-speed serving machines and do not apply the serving on as tight as it should be, and they don't take enough time to pull the tag ends tight on the back-serving knots. This will lead to end-loop serving separation and center serving separation.
Some string manufacturers/builders like Zebra strings don't serve the end loops. I am not a fan of this. I prefer to serve the end loops as it keeps the strands from separating and getting mixed up and it makes it a lot easier to install the strings. I think there are better strings than Zebra strings. I am not saying they are a bad string, just there are better for the money.
I do have a serving machine that I built myself and it does make the building process faster and easier. But I don't serve very fast as I want to get the serving tight and stacked with very tight back-serving knots. I serve with the string under the typical 300 pounds of pressure, and this really helps to make a stable string. I also take some steps to ensure that the string is completely stable before serving.
I usually just do local work and I will build the string/cables, install them and tune the bow. I get to look at the old factory strings on the bow and see how they held up over several years. I can see where the "trouble spots" are and make sure I address those areas. I measure the old string/cables and see how close they are to spec and if there was too much creeping. I can examine the serving and check the quality and knot tying quality and the size/type of serving used. I can check the diameters used and adjust the diameter on the new string/cables to give better results.
In the last month, I have rebuilt 6 Ravin crossbows, a Mission crossbow, a Mathews Switchback, a Hoyt Alphamax, installed a set of 60X string/cables on a Bowtech Realm. It's been busy.