It's probably not difficult to extract Tor encryption keys from storage or memory on virtual hardware like EC2 instances. Similar issues likely exist for rented physical servers with management access.
Does this make running Tor Cloud bridges less advisable than running a bridge or relay on your own hardware?
Yes, in a sense they are less secure since it's probably trivial, given sufficient access, to copy the system. Real hardware requires physical access and is, perhaps, easier to detect: an extended and/or unexplained downtime would probably be noticed.
However, diversity is very important for the Tor network.
Therefore, if the choice is between a bridge or relay on a virtual/cloud service on the one hand and no relay on the other hand, I'd argue that having the relay wins.
