A PFL means IRCC intends to refuse your application. So yes, it's a bad sign and you must have a strong response to the PFL to avoid refusal. There is also the possibility IRCC may refused the application and also give the applicant a 5 year ban for misrepresentation.
As said in the response above, IRCC does not release approval / refusal statistics for applications that have received PFLs. So success rates are unknown.
