There are ups and downs both ways I think. If you use your pen name you're a writer reviewing fellow writers. If you do a good review then you're accused of kissing up to your writer friends. If you do a bad review you're accused to trying to do down the competition. You also have some genuine conflicts of interest if you are reviewing books that are from your own publisher, or from writers who
are your contacts and friends. The pen name also of course links your reviews to your own books on Amazon and that invites possible retaliation for a bad review, from a writer or a crazy fan of theirs.
On the other hand if you use your real name or some other handle, and people figure out you're such and such a writer you'll be accused to trying to hide your identity (by using your real name...
) so that you can bash the competition without risking having your books bashed in return. And like you say, there are many crazy people on the Internet and if they take offence at one of your reviews they can start all kinds of trouble.
I've pretty much given up reviewing anything in my own genre! It's not worth the potential trouble.