I don't know of any hard limits in terms of returning results in general, but at present it does have a reviewable item limit
Depending on criteria of the search criteria, it could result in an appliance running out memory resources allocated, an example would be a complex search with wildcards. In this situation it'd be best to contact support as we can in some cases tweak the appserver resources to accomodate, the alternative is to split up the search and batch the results.
Wildcard expansion does also have some defined limits, this would present an error on the results page, see below:
Hope this helps a bit.