Actually, "hit-and-miss" is incorrect in this situation. From this English Language Usage question (emphasis mine):
Hit and miss refers to multiple tries.
Hit or miss refers to a single try.
In this case, if the exam can only be taken once (which I am assuming to be so), then it would be "hit-or-miss".
The definition of hit-and/or-miss, from TheFreeDictionary is:
Sometimes good or successful, sometimes not; having mixed or unpredictable results; random, aimless, careless, or haphazard. (Hyphenated if used before a noun.)
while the definition of touch-and-go, also from TheFreeDictionary is:
Extremely uncertain as to the outcome of something. (Hyphenated if used before a noun.)
The main difference is that "touch-and-go" is for an event that will always be unlikely, so in context to the question, it means that it is very unlikely for Ben to pass this exam. "Hit-or-miss" in the same context would mean that it is random and unknown whether Ben will pass or not. Just remember the difference between "hit-and-miss" and "hit-or-miss".