So as you may know, I've done a number of FOBT games. I can share some info..
FOBTs that are advertised as random ARE truly random. The RNG that many people use is this one: http://www.math.sci....mat/MT/emt.htmland the seeding is done variously, but I chose to use the number of microseconds since the PC started. As this will vary every game load, it is random enough so as not to be predictable.
All UK FOBT games must be certified by a Government approved test company (also known as a "test lab") and we pay quite a lot for them to do that. They're all PHD level mathematicians. They get the entire source code stack, RNG (which is a DLL, and never changes, anyway), a set of simulation results that they can then verify against their own tests, and a fully compiled and working game version for both PC and on-cabinet. They have a full cabinet and back-office system to replicate the live use of a game.
After about a month, assuming no issues are found, then the test lab issue a set of signed certificates which allow the game to be made available to the public. Those certificates can be inspected at any time by the GC and have to be kept on record, and under lock and key.
Any update that affects the mathematical parts of the game require the game to be re-tested.
Any company that does not comply with the above rules, firstly won't get a game live because the platform operators simply won't accept an uncertified game, and at each company (developer and platform) one person is a named licence holder with the GC and has ultimate sign off (and therefore, ultimate PERSONAL responsibility) on the game to be put live. People don't disobey the rules as you are personally liable - you can end up in prison.
BTW, you can do a compensated FOBT game but they've never really been very popular. Project tried a few, I think.