Charles, one of the big advantages of GNU/Linux over various flavors of UNIX is the open source of the former. Since the source is open and anyone can play with it, it is much easier (and cheaper! My UNIX license cost me over $1500.) to make changes to the base system to custom hone it to a fine edge for one's own particular needs.
This was the big advantage of Linux variants over UNIX variants for the rendering companies: They were able to customize the entire OS for their particular needs rather than using a general purpos OS - tremendous increase of speed.
Banks used to use a lot of different OSs for their base - many used Pike, which was an odd variant of UNIX System III redesigned for financials. It was specificly for multi-processor machines and is a fault-tolerant system that is constantly comparing the output of the processors all doing the same job. If there is a disagreement the calc is done again until the majority of prosessors agree. Tandem also had a similar UNIX based system for banks and ATMs. Both were quite slow but incredibly reliable.
That is changing now and many functions are being moved to general purpose OSs.
Worst of that, currently, is many ATMs are being moved off the dedicated high-security OSs onto MS Windows XP and customers are now seeing the BSOD and MS Windows errors at their ATMs (especially Diebold).
(Side Note: The first UNIX system I bought ('85) was a UNIX III based OS made by SCO ... Most of my UNIX exposure was to BTL UNIX at Bell Labs and AIX and BTL UNIX at the operating companies. These days I'm running a BSD based UNIX, though I still have my old 1985 SCO running machine, and it still works fine ... )
In 1972, UNIX was the best thing since sliced bread, but in the over thirty years since then other things have come along that may be better. GNU/Linux was originally just a 'clean room' reconstruction of UNIX functionality, but has grown waaaaay beyond that with thousands of people working tirelessly on it. Because of being closed, the UNIX flavors have not kept up.