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Sorry to be so long getting in on this.
Several posters have addressed the subject of immediate fractures. As mentioned, these can be caused by:
1] too deep a cut: I try to keep it to no more than 1/3 the thickness of the stock, or 2mm, whichever is less,
2] too wide a cut, which does cause the cutter to be sweeping across the grain at a right angle. In particular, make the first pass shallow; say around 1/8" in from the edge, so that the stock is backed up, or run against a fence, and,
3] setup issues. These could be; a dull tool, or cutters that are set at different heights. I'll note that I got one Wagner planer that had warped cutters, and the bottoms of the pockets weren't flat, so they could not be set securely. An hour or so of lapping the cutters and scraping the pockets sweetened that one right up.
This does not seem to be the issue you're really interested in, though: microfractures are ellusive by nature.
I don't have hold down rollers, and have not missed them for the most part. I do find there is some surface tearout, particularly on figured wood, and conceivably that could extend deeply enough to do damage if you violate The Rules above. Obviously, for example, it's possible to tear apart a piece of curly maple or ebony veneer if you take too heavy a cut or whatever. In general I find that leaving .2mm extra thickness to scrape off later is sufficient, or maybe just a little more on highly figured wood. But, again, that's all about visible damage.
I'm not sure how to address the question about hidden damage without doing some science of a fairly time-consuming sort. You'd have to thin out some pieces of wood using both a sander and a DPP, glue them to frames, and run a lot of moisture cycles or other kinds of stress, and see which has the higher failure rate.
I've used Wagner planers for thirty years, and don't beleive I've had abnormally high rates of cracking, say, at least since I got my humidity under control. But, of course, that's just anecdotal evidence: what's a 'normal' rate for this sort of failure, and what about the folks who are so disgusted with all the cracks that they just don't speak up? Without anything to compare my experience to I can't say.
It does suggest that the effect is 'small', if it exists, and that you would need a fairly large sample in any real test to see it. Only you can decide whether your level of doubt warrents that much effort to ausuage. Is it likely to be any greater than the effect of, say cutting logs with a chain saw or ripping stock with a tapble saw?
In that regard, there is one place that might have some answers: the Forest Products Lab. There is still a lot of wood being used in aircraft structures, and more was used sixty years ago. They did studies on things like sanding vs planing glue joints, for example, and if ripping with a circular saw could conceivably damage wood structure enough to threaten the integrity of an aircraft, you can hope they'd be on it.
Other than that....
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