What would constitute an "unreasonable" internal link structure?
The value of external links is distributed from each entry point - and no site of any size has external links to every page.
So, the "job" of a good link structure is to distribute the available benefit in a way which:
- Reflects the relative importance of each page (there's no sense throwing as much weight at a lowly review page as at a top level category)
- Ensures each page receives "enough" benefit. "Enough" depending on the role of the pages. As far as Google is concerned, this means "enough to rank" - which might be as simple as not getting binned as low quality.
The two most common problems I've seen with link structure reflect those two points. Or to put another way, unreasonable link hierarchies can:
- Waste link juice on lowly pages that don't need that weight. This results in lower rankings, especially for competitive areas.
- Don't provide enough links, or enough link juice left once they get to longer tail content. This results in pages being regarded as of low quality.
To "visualise" somewhat, there are plenty of smaller sites that don't have well designed navigation, and rely on a large HTML (or even XML) sitemap. These sites tend to have a handful of very strong pages, linked to frequently, and then the rest are very low value, since all their juice arrives from a sitemap. This also occurs on larger sites, but tends to be a series of smaller "sitemaps" or pages with large numbers of links to content that isn't linked elsewhere. I sometimes call this the "running our of menu" problem ;)
Another example would be sites with huge drop down menus, that link to everything from everywhere. This results in equal weight being given to pages at the top and bottom of the hierarchy - when the distribution should instead follow the relative importance of each page. This is the "too much menu" problem ;)
Information hierarchy as it applies to internal links and navigation isn't the simplest area to get your head around, especially when lots of pages are involved. Hope that helps a little though!