ZNC has its own issues. We had a repeatable crash that happened every time a particular user connected (it took us a while to figure that out).
I do not claim ZNC or our whole IRC setup was perfect, but for me and many others, it did a very decent job of providing effective, no-frills, getting-things-done means of communicating, all while consuming little resources on both server and clients, and being simple, underestandable, and easy to hook other software into.
I really miss it, and personally dislike (especially the client UX of) most trendy/recent alternatives.