It also really depends on the drives used. There is no way to securely delete stuff from a SSD except for initializing a "secure erase" which will wipe the entire SSD. And even then you have to trust the manufacturer that the firmware will really delete everything. I personally like to encrypt everything. If you don't want a VM anymore, you can delete it and throw away the key. The encrypted data is then still on the storage but no one can make use of it as the encryption key doesn't exist anymore.