What are the differences between struts and units? 

A warm up question. Units are static objects that exist from the start of the simulation right up to its end, whereas struts are formed on the fly and automatically destroyed by the garbage collection mechanism when they cannot be accesses anymore. Units are used to store data that is needed by many entities, because it is simple to know where they are and reach them from anywhere in an environment (for instance using such unit specific methods as get_enclosing_unit(). They are also used to connect to DUT interfaces (BFMs) because these interfaces are active from reset to reset (which is normally equivalent to from begin to end of simulation), and for other fixed environment elements like scoreboards.