He also said that the various Badatz hechshers generally only go into a place and declare it kosher after the Rabbanut has already done so. The Badatz cannot be relied upon in a place where they declared it kosher without first the Rabbanut doing so, according to Rav Ariel.
source: Srugim
So, according to Rav Ariel, Hashgacha Pratit is just like any other hechsher - just like any private Badatz, for example. According to Rav Ariel, only the Rabbanut can give kashrut and be trusted, and no private hechsher can.
Legally that is the case, and will be so even more so after Shas's new law will be passed in the near future. Halachically that is not the case.
According to halacha all you need is to trust the person telling you it is kosher. There is no concept of hashgacha, private or public. I can trust the Badatz privately, without needing the Rabbanut, from a halachic perspective. I do not even need the Badatz if I trust the owner of the restaurant, or if he hires a private mashgiach that I trust.
That being said, as I have said before, I do not really get how Hashgacha Pratit works. At best it is like any other private hechsher, as Rav Ariel said, and one can choose to trust them or not. In reality though they are trying to do things differently. They have some system of volunteer supervisors, but for the most part people are relying on the word of restaurant owners who are largely not religious and whom they generally do not know personally. Can they be trusted? I don't know. Anybody and everybody can decide for himself or herself, but to me it seems spurious. Even though in theory I like the idea of going back to the system of trust that kashrut is really based on, I do not see how it can work the way it is working.
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