If you take the time others need to spend with them getting them up to speed away, I generally consider it good if they break even in the first month.

That's a very one-sided metric. There is a large range of definitions of large and complex. The dev process or team size might make it extremely difficult for outsiders to contribute. I can understand why you and others would like to attribute all productivity squarely on skill. In almost all cases, everyone is trying to manipulate the consensus of who's productive and who's not, who made a good hire, who has good employees, etc.

It might as well signal of an over-complexed project design.

Some systems are needfully complex, because they do a hell of a lot.

And some are needlessly complex but you inherited them from another, long-gone team and somebody has to maintain, improve and update them, unfortunately!