Some of Freddie Mac have a variable rate, that is, they "float" based on some index, say Libor or two-year Treasury bonds. Not saying you should avoid them, but if the dividends were ever restored, these preferreds would play a low dividend based on current market conditions.
The prospectuses for the preferreds are available here, just hit the link for the one you want to look at.
The preferreds have a redemption value of either $25 or $50. I believe they are all non-cumulative, meaning that the dividends that would have been paid during the September 2008 to present period are gone forever. Our preferreds are non-cumulative, which makes them quasi-equity, while cumulative preferreds are more like quasi-debt. By being non-cumulative, quasi-equity, they could be considered "core capital." The preferreds were "Tier 1" capital, which made them a favorable investment for commercial banks and similar investors, prior to the events in Sept 2008.