I've ended up use gerbv after having used several others including both open source and proprietary "free".
gerbv is not without it's quirks, but it is so simple the "issues" are few and mostly to do with the file contents not being as it expects. I won't say not standard as I don't think there is a standard. The Gerber files are defined in a spec, but the drill file is still only defined by the company Excellon I think. Well, I looked it up and while it is not part of RS274X, the Gerber file format, it is a separate IPC standard, IPC-NC-349, a superset of the Excellon defined format. The issue is that no one seems to use the format 100% correctly. I have some files that it barfs on. When I dig into the problem it's a bad file. But most Gerber viewers can work with it.
Maybe the newer versions work around this bug in the file. I don't know, i haven't created any drill files with that bug lately. I don't even recall which program does that.