I became a non-licensed operator after getting my Mech Eng degree 20+ years ago - I would make that move again without hesitation.
Pros are you get lots of initial & continuing training - much more than other positions. The NLO position is probably union or at least non-exempt so you should be paid for your OT, and probably get premiums for working nights, Sundays, Holidays, etc. Also working for a utility should be very stable if you choose the plant wisely. At my plant the ops career path pays MUCH better than the Engineering path. (In my first year as an NLO I made ~50% more than I would have as a new engineer). Make sure you speak to an incumbent about how much either job REALLY pays.
Downside for some is the shift work schedule (nights weekends & holidays). Also all of that training comes with a lot of testing, and your work is often observed and critiqued (more so once you are licensed), and finally the more time you spend away from engineering the more you risk losing some of your technical skills you just worked so hard to get.
Note that with the engineering degree you should qualify to get an instant SRO license IF that interests you. The Ops path is a much more certain path to that goal than any other and an SRO license IS your ticket to further advancement in commercial nuclear.
Whether its easier to go from ops to engineering or from engineering to ops - that will depend on your organization, their demographics and philosophy - but Ops is generally a pipeline to the rest of the organization, not the other way around. I don't think Engineering would offer the same level of career opportunity.
That's my 2 cents