So, "systems integration" job involves making systems interoperate as seamlessly as possible, by doing so adding new features, or reducing weaknesses, like for example vendor lock-in (sadly this is not always possible in industrial automation, see below). So the concept applies also to other sectors - and there are indeed non-industrial automation SIs - you might want to add terms like "PLC", "SCADA", "ICS" (industrial control systems), "industrial automation", "controls" to your search.
As for my own experiences in the (industrial automation) system integration industry, in Italy:
As I said before, and said elsewhere by TheCapn, vendor lock-in (in the form of employing equipment by one company, and their tooling, and their services, etc.) is a thing. But at the same time, your job is to make heterogenous systems interoperate! So most certainly you'll have to work not with your usual tools. Personally I have found that the most successful companies rely on some vendor (and their support, and their expertise), but are very open with working with, and know, the others. Obviously the more you can develop autonomously the better; it is a trade-off between freedom of action and access to more credible (branded, battle-tested, supported by big company etc.) solutions.
Knowing when to add one part to the system, or where you can just expand one of the already existing parts in order to accomplish the task is fundamental. Too often I have seen superflous parts added where it would have been enough to extend one of the existing parts a little (This is often due to vendor lock-in, you cannot work on some equipment, so you put another device by your vendor of choice).
Another thing I love about my job is the "Jack-of-all-trades" mindset required. Also human skills are important, especially here where the industry is mostly composed of little shops: sometimes you just have to understand enough to call the right specialist and contract that part of the job to them. I started as more of a computer person than a engineering one, but I managed to gain some expertise over time thanks to this knowledge transfer.
