PSA: Unlike mechanical relays, most solid state relays are made to switch only AC voltages. Which means that you shouldn’t use one to drive a 12V or 24V (DC) heated bed. A proper MOSFET is a much better choice here.
SSRs are typically designed to minimize switching losses by only switching off (or on) at the point where the switched voltage inverts its polarity - also called zero cross trigger or zero-crossing detection. Since a DC supply never inverts its polarity (or even drops to zero), the behavior of the SSR may become unpredictable, ranging from the SSR never turning on or off or the entire SSR melting down.
Also, SSRs have a comparatively high voltage drop across their output (Fotek states 1.6V), which causes high losses when switching large currents and reduces the available power at your heater.
Not quite true. A SSR employing a triac will exhibit the behaviour of turning off when load current approaches zero but, unless it has a zero-crossing detector to control switch-on (which a lot do not have), it may be switched on at any part of the AC cycle. This can have issues for e.g. inductive loads which object to the application of a DC bias. Here you must ensure the switch-on occurs at the same point of each AC half-cycle.
Any good articles comparing the performance of MOSFET vs SSR vs other types of current switches? I’d like to see how they perform in terms of efficiency, heat, etc. I have a RAMPS 1.4 board and love it, but it seems like a good idea to move power off that board where possible, and SSRs seem to be an economical option. But is the original post correct? Are SSRs a “hack” when used with DC current?