That's the impression I got too. Maybe I should clarify why I said that about the pro-black creatures. In a typical scenario against a white deck, the opponent won't be using solely pro-black creatures (otherwise he's going to look pretty silly against a red deck or whatever). At a given time, there might be one or two on the battlefield, maybe another one if the opponent has creatures with variable color protection or something. This is not necessarily lethal. This deck can still take out the opponent's other creatures and then "ratbomb" the opponent to death, doing more damage with Crypt Rats (one-sided or even with life gain thanks to Basilisk Collar) than the opponent's White Knight or whatever can dish out. Now, it's also possible that a fast white deck will deal massive damage before the combo is online and throw the last damage in past this deck's defenses thanks to protection from black. I thought that was Mooseman's concern, but it's also possible he was talking more generally. Either way, for cases like this, I think Crypt Rats deck is realistically going to lose anyway. Trying to patch the deck to compensate for this one weakness doesn't seem worth it. Then again, maybe I'm just not thinking of the optimal ways to do so...
In contrast, anyway, a similar deck that plays Circle of Protection: Black (or any similar card) can slow the "ratbomb" damage that the opponent takes down to a crawl, leaving plenty of time to win with a single pro-black attacker (or something else entirely). With one card and a little support, this deck can be dismantled. CoP: Black isn't even the best card to do this, but it's one that is, or used to be, popular in casual games, which was why it was the example I used. I should note that I've built and used plenty of decks that can be dismantled by one card (or one card with a little support). I mean, any deck is going to have some weakness. Whether this one is particularly glaring depends on Sentinelking's goals for this deck and on what sorts of decks he finds himself playing against. All I can really say for sure is that this issue is the one that would leave me hesitant to build such a deck, were I contemplating doing so.
I suppose that's also based on experience that's pretty well outdated now too. Lately, when I have played Magic at all, it's been with Legacy tournament decks that weren't really what I think of as casual.