I have three sets of rails in use.
The first is a set of 2nd generation Festool rsils (1 x 3m, 3 x 1.4m and 1 x 800mm) which replaced my original 1st generation Festool/Hilti rsils about 12 years back. They get used with my two Festool corded saws. There is very little variation between these rails where they join
The second is a set of Makita rails (3 x 1.5m and 1 x 1m) which get used for the cordless DSP600. Again, no noticeable variation between rails.
The third set is a budget set by Evolution (2 x 1.4m and 2 x 700mm). They were bought for use with a secondhand Bosch GKT55CE cutting cement fibreboard fascia and flooring. On those, whilst the profiles are the same, two ot the rails suffer from a bit of twist at one end which means that the saw "bumps" a bit as it goes over the joint. In view of the application, I am not unduly worried by this, but I'd be less happy if I was using them to cut wood panelling.
I have used other people's stuff over the years as well as my own (mainly firm's stuff), often with two joined 1.4/1.5 metre rails, but never noticed significant differences. A quick straw poll taken at work yesterday, where we have one more Makita user and two Festool users also revealed no issues with guide rail profiles. My feeling is that this discrepancy is a where a single sub par item (possibly from the beginning or end of a run) has slipped through the QA net, but that both rails should be returned to the vendor and replacements requested. These things do happen, but I don"t feel that changing to Festool rails would make any real difference, and it would lose you the Makita's USP - that tiny lip on their rails, which combined with the retractable tab on their saws, allows you to do full 8ft bevel rips without the saw trying to tip over anf fall off the rail for the whole cut (significant if you do a lot of bevel rips)
As an aside, back in the late 1970s, when I was between jobs (the construction industry can be a bit like that), I did a short period working for an aluminium extruder, on the production line. The process is that billets of aluminium are heated in an oven then dropped into a hydraulic press (which to all intents and purposes resembles the breech on a battle ship's guns). The hydraulic ram then squeezes the hot aluminium through a large steel die causing the profile(s) to exit the die at speed. Part of the trick to getting a consistent extrusion is apparently to run the press at a consistent pressure (and therefore speed) with as few stoppages as possible (every time there is a stoppage, the restart can cause a mark, blemish or even a change in thickness which needs to be cut out and scrapped). The beginning and end of each extrusion, which were up to 30 metres long, was generally pretty ropey (thin, bent, etc) especially on complex "weld back" profiles, so would be cut away with a gas torch and sent for recycling, but the majority of the extrusions, once cut off, were pulled across the tracks towards the saw station, cooling as they went. When they reached the cutting station the saw man would cut extrusions to length per the job sheets and stack them in trolleys. Obvious flaws, such as marks, variations in thickness, twisted or bowed pieces, etc were again removed at this point. The last stage in the process was to anneal the completed and cut to length profile sections overnight in an automatic oven. I was told that this was to destress the extrusions and that it also reduced or removed any minor twists, etc.
What did surprise me was the amount of human judgement there was in the process, especially from the press operator, and how little (relative) waste there was.
