I'd be not so much cautious as merely aware that a book of a given vintage can't be expected tp redlect new findings from intervening research, but if it's worth its salt represents rather a presentation of the scholarship of the time that may need to be read in conjunction with later works.

The publication date itself I don't see as a criterion: despite considering myself a thoroughly modern social scientist I routinely draw on sources several times older for evidence, insights or approaches that may since have been overlooked since or reworked, summarised or in cases garbled almost to extinction even as they're invoked in support of new work. You can't study France's pre-revolutionary economy without Tolosan (1789) or England's a century earlier without King (1696), despite having to adjust for considerable deficiencies in each given the contemporary shortfall in data availability and modern quantitative technique.

Just to take the first chapter of the second of the late Qing volumes of the CHoC to which I believe you may refer (in this case from 1980), I still dip not only into Feuerweker's contribution on economic trends but also his 1970 article on cotton textiles, Perkins's 1969 Agricultural development, Liu & Yeh's 1965 national income study and probably most of all Buck's classic 1937 farm survey, all cited in the text and still quoted today despite each requiring a degree of reappraiasal.

Newer frankly does not always mean better: while offering us a wealth of fresh perspectives or data, academia's institutional "compulsion to publish" (sometimes "publish or perish") has also yielded works which in more demanding times would have been returned to the creators with a helpful note of "Must try harder". Even authors whose previous work I admire hugely aren't above the odd rushed filler to plug a publication gap on the cv.

And it has to be said that scholars aren't necessarily replaced any time soon by someone of their calibre, even as new knowledge emerges to supplement their work or offer new lines of enquiry: I can think off the top of my head of half a dozen still active experts in their field whose shoes show no sign of being filled when their work ends, despite it probably having never been easier for prospective stars to exhibit their wares.

Nor can it be denied that there exist approaches that belong to the realm of fashion rather than timeless advancement of knowledge: I'm referring not to valuable recent more inclusive incorporation of previously under-represented perspectives, but rather to fads such as the widespread meaningless effort to reduce past economic data to some imagined present-day common monetary base, a personal pet bugbear of mine that I don't see as likely to stand the test of time.

Finally, remember historians are a contrarian bunch among whom nothing succeeds like overturning past assumptions. But to understand the debate and determine who's right or whether productive babies are being thrown out with superseded bathwater, it's necessary to see how the earlier notionally-discredited viewpoint arose. Even to demolish a case, you have to return to the source to know what it was, and why it came to be so.

So I'd read that ancient 1978 scholarly text without fear, but just appreciating that there are another 42 years of scholarship to be drawn on which may or may not challenge its conclusions. History is a cumulative science, but that doesn't mean past works are intrinsically flawed in their findings or analysis, just that there may be more to add.

To add to what /u/davepx has said, you also need to be aware of major paradigm shifts in the ways that certain periods are understood. A rather unfortunate consequence of the relatively early publication of the two Late Qing volumes (10 and 11) is that they predate the New Qing turn of the late '90s, which can be understood as almost a complete overhaul of our understanding of the Qing, drawing attention to matters of ethnicity, trans-Eurasian parallels, and specifically Manchu features of rule. By contrast, the two early/high Qing volumes (9.1 and 9.2) are from the beginning of the millennium, and include contributions from major 'New Qing'-paradigm scholars like Pamela Crossley, William T. Rowe and Nicola di Cosmo. So while the CHC on the Late Qing remains acceptable as an overview, if it were written today there would likely be more acknowledgement of what we now understand to be key dynamics of the Qing period.