Overall this product should appeal to fans of OSR megadungeons. Forbidden Caves of Archaia is pretty well known in OSR circles, so its only natural that it was eventually converted to 5E. I saw this campaign on Kickstarter and immediately backed it. Seemed like a no-brainer.
Now for the problem that led to knocking the rating down from 4 or 5 stars to 3 stars. The monster stat blocks are very, VERY incomplete. Dozens of unique monsters have no stats at all and the DM is just instructed to "see Barrowmaze Complete". In effect, you're required to purchase an additional $35 PDF just to properly use the one you just purchased.
A DM can of course substitute any monster that they wish, but they are still given no help in the form of a CR rating or any other info. Just a name like "Juju zombie" or "Gemstone golem". Not very useful. The added prep time from poring over every chapter and plugging in your own monsters is both time consuming and drains the product of the unique flavor that you paid for in the first place. The stats obviously exist somewhere and this was almost certainly a cost cutting measure designed to keep the book's page count down and affordablility up. That's fine. But here are some possible solutions.
-Provide a "Recommended Monster Replacement" table. Just list the unique Barrowmaze monsters and the author's recommendations for OGL or MM monsters replacements. This would require at most 2 additional pages
-Provide the missing stat blocks as a free PDF here on DTRPG. It could be completely free or free just to verified purchasers of the full book. Kickstarter backer like myself would be out of luck in the later case, but it would make new purchases of the Forbidden Caves of Archaia worthwhile.