I keep my sealed tubes in my humi. Never had a problem with any of them. There are vastly different tubes you should note. Many metal or plastic tubes were not intended to be a perfect seal, Others like the wax sealed glass tubes, do a heck of a good job with the seal. I tend to open the metal tubes, but leave the wax sealed glass tubes as i got them.
The mold problems I've seen with tubes often have to do with condensation forming inside the tube from dramatic changes in temperature. Down here is Florida it's a problem, a store has a nice cold storage cabinet that keeps being opened by customers in a hotter humid room. The warm blast of air hits dew point and you have wet, alum tubes are the worst. Once in my humidor I've never had a problem with any tubed cigar.