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]