In yacc the generator will tell you about shift reduce conflicts. Once you have debugged the grammar the parser is likely to work. With parser combinators you have no such assurance - you don't know if your grammar has loops, the parser might get stuck easily while parsing. However for a simple and regular input language like sexpr everything is fine.
you can have left recursion, or implicit left recursion in your recursive descent grammar, if you have then the parser gets stuck while parsing a clause that contains left recursion.
Firstly, this have nothing to do with shift/reduce. Secondly, you can safely handle left-recursive grammars in Packrat (which, in turn, can be implemented with combinators).
I see, yes, you mentioned loops earlier. Anyway, it is not a problem for combinator-based parsing, just use Packrat with a left-recursive extension [1]
