No experience with Nappynex, but similar products usually are satisfactory for light incontinence. The Velcro-type fasteners usually are quite secure. Do note that with Nappynex your absorbency is in the crotch and not in the sides. If you are a side sleeper that would give you little protection. The product seems designed more for daytime use.
Although your bedwetting is "occasional," the more important question is "how often and how much do you pee at night." The Nappynex might not take two full wettings and hold it without leaking.
However, for night use (I am a bedwetter, sleep on my side, and use cloth diapers), you would have better results from a regular night-weight adult cloth diaper with plastic pants or PUL pants on top.

I fasten my cloth diaper with pins as I achieve a tighter fit than with a Velcro-type fastener.
If you get the Nappynex, please let us know how it works for you.
--John