For initial certification all that was needed for most light twins was to demonstrate a one degree climb gradient at 5,000' PA at ISA with critical engine feathered and at AUW.. Always achieved by test pilot in brand new aircraft after many practice sessions. Having said that some light twins did it easily, others not so much.
The PA-31 series outstrip this requirement easily if loaded at light to mid weights. Again notwithstanding poorly maintained aircraft or pilots with poor technique, even the 40 year old machines will still perform if kept within limits. The only issue is close to max weight (or above) in low density/hot conditions.
No aircraft type will perform under the right circumstance of poor maintenance/poor technique or multiple failures involving damage. Atlantic Southeast Airlines Flight 529 is a good example of a transport category aircraft engine failure that led to a situation that the aircraft could not maintain altitude.
As for fuel bladder issues, this should be pretty easily covered by proper maintenance and disciplined fuel checks and records.