The security of a blockchain network depends on how many nodes are participating in that network. More number of participating nodes, implies better security. Currently, many financial institutions are integrating a "private" blockchain into their operations. Since the number of participating nodes in a private blockchain is likely to be far smaller than the public blockchain, does it mean that it is easier to tamper the private blockchain? If that is true, then does it imply that ad hoc private networks of blockchain adopted by financial institutions will be less secure than a big public blockchain network?
this has the risk of getting a philosophical debate... Generally, the bitcoin blockchain was invented, to avoid, that malicious participants have no impact on the overall distribution of messages in the public network (aka tx with Satoshis in blocks). Along with some privacy features. So when someone creates a private blockchain, we are down to the level of trust. You can use a 1:1 copy of the bitcoin network, and don't go public. Then you compare the code to the executables, and can be sure, noone wants to tamper with it. But as it is private, you never know, how many "good" and how many "bad" nodes were invited to participate. There are use cases for private blockchains, when it comes to tx handling between several companies, which should be tamper proof. Question is, if the bitcoin blockchin is the right tool for this. In general I support your point of view, that private blockchains are less secure than a big public network. Trust is then based on the fact, that this company will not go bankrupt (or controlled by governments), and the privacy question remains unanswered, of course.