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Two other problems with cards like these are:

(1) Points you collect in their program can't be combined with points from other sources. You have to earn awards solely through charging with that card. If, like many other people, you also travel a bit and earn miles/points in airline/hotel programs, you'll end up with more, but smaller, piles. These can be harder to use for rewards than one or two big ones. That's why most people around here tend to prefer cards that give them additional miles/points in programs where they're already earning them in other ways.

(2) Some of the highest-value redemption options involve buying a cheap economy fare to somewhere, usually another continent, and then using airline miles to upgrade. Since these cards work by having the credit card company buy your ticket and airlines don't sell upgrades separately, this option isn't there.

(3) Along similar lines, premium-class travel awards are also good values in most airline programs. They're generally far less expensive in miles, relative to an economy award, than they are in money: on the order of 2x miles, where they might be as much as 10x or more in $$. Since in this system the credit card company pays the airline in money, premium-class travel is generally out of the question.

This is not to say these are evil. If you charge a lot and want economy class travel awards, they can work well if you read the fine print. (They say "no blackout dates," for example: technically true, but to popular destinations at popular times tickets that fall within their price restrictions are likely to be unavailable even though the date is not explicitly blacked out.) There's a lot more about them in the credit card forums (fora?).