Are server motherboards really less reliable?

If you look at server board reviews on Newegg, etc., they either have no/few reviews, or the reviews are pretty bad, on average: 2-3 eggs out of 5 (I'm not counting the products with just a handful of reviews, where the average score is very random and unreliable)

Are server boards less reliable in general, or is something else at play?

Good server boards are going to be very reliable. You can't go solely on NewEgg reviews other than to get a general idea of a board's failures (if tons of people are reporting the same thing). People are much more likely to complain when they have an issue, rather than praise when it works.

Quality will vary between vendors and anyone can label a board as a "server board". What are you looking for?

Good server boards are reliable as they're passing more tests than desktop hardware. The only problem is that in last 2 years we see that some manufacturers are saving on components while prices aren't low.
Intel is no longer selling motherboards under their brand and maybe it's good as their last boards had many issues. Last one that I was testing had BIOS problems and Intel couldn't solve it for over 3 months+.
I've never had any problems with DELL or IBM servers. There were some issues with Fujitsu.

When you find a board with all features that you need then browse for comments about this model on the forums and check BIOS list and other support documents. Newegg comments/reviews are one of the worst place to look for a good info.