PNP / NPN Reverse current and voltage (Vce/Vec and Ice/Iec)
Most jellybean silicon BJTs have much worse characteristics in reverse. Beta is much less and breakdown is more like 8 or 9V. The characteristics are not guaranteed or even given as typical on the datasheet because most people do not use them that way. Since they makers won't sell any more parts by specifying those parameters they don't bother. 

You can sometimes find the reverse beta estimated in the SPICE model, and a (usually conservative) limit for reverse Vbe is almost always given on the datasheet (5V is common, but lower and higher voltages are seen). 

There are (or were) transistors made with better characteristics in reverse, or even made to be symmetrical. Some are aimed at the analog mute function, and in the old days some were used for analog switches. They are not as good for most applications as the more common asymmetrical BJTs.