nitrocellulose isn't necessarily very stable. If it's an amateur making gun cotton there's always the risk that they won't properly wash out the acid and if there's enough left, when left to dry the evaporation of the water can concentrate the remaining acid enough to cause spontaneous combustion. Other than that it's pretty stable but worth pointing out that film used to use nitrocellulose as the base material and there's tons of movie theatres that burned down in those days and even just vaults storing the film under ambient conditions. As the nitrocellulose decays, it can release nitric acid, thus accelerating the decay and potentially starting a fire.
If properly prepared and used and stored responsibly then gun cotton can be safe, even when used for teenage antics. Lighting off a piece while resting it on an open palm of course being one classic trick with nitrocellulose. I'd say storage and transportation of low and high explosives are some of the riskiest aspects because there's so many insidious ways for it to go wrong that you wouldn't learn about until you need to take your shoes off to count past 7. It doesn't help that nowadays it's so trivial to find out how to synthesize acetone peroxide and storing it long term is always going to be fraught with peril. Especially when someone without any semblance of proper containers stores it sealed up in a plastic bottle with a screw on cap. Just one drop or one crumb of primary explosive on the screw threads and maybe you get it sealed up, but when it dries out in the threads there's a good chance it goes off in your closed up hand when you try to open it and add in all the friction rubbing between the threads.