I can not think of much for a downside for illuminated nocks. I use Nockturnals, and the only issue I have had is with the switch. Once or more used Nocktunals do not stay off sometimes. I have opened my bow case to find one or more nocks on, or would get to the field and hang my quiver only to see one of them on. Now, once used Nockturnals are downgraded to my practice arrows. I have never had another issue with them. The fit is tight but good, some tighter than others but all come out with some twisting.
To operate the switch, I have tried both the old version and the new one made by Nockturnal, a knife, and a field point. The switch does get very chewed up on my practice arrows where I shoot them at relatively short distances and operate the switch frequently, less so with one of their tools made for this purpose.