I think it boils down to that it doesn't matter what they promise, if you send a videocap of all you ever do on your computer to some company on the internet, you just have to take your chances. Would you put mics and cameras in all of your rooms in your home that send data to Apple (or someone else) to analyze "for your benefit" even if they say and promise they won't do anything bad with the feeds?
At least with gmail and chat clients etc. things are somewhat put in compartments, one of the services might screw up and do something with your emails but your Messenger or WhatsApp chats are not affected by that, or vice versa. But when you bake it into the OS (laptop or phone) you're IMHO taking a much bigger risk, no matter what the intentions are.
There is nothing which Apple Intelligence can do that a hypothetically evil Apple couldn't have done before, given sufficiently treacherous code in their operating systems. Thus if you use an Apple device, you're already trusting Apple to not betray you. These new features don't increase the number of entities one must place their trust in.
Whereas with apps like Gmail and WhatsApp on an iPhone, you must trust Google and Meta in addition to Apple, not in place of Apple. It doesn't distribute who you trust, it multiplies it.
I still think it's a big difference between trusting existing OS'es and apps, which are under scrutiny by hundreds of security researchers and thousands of security nerds all the time, and willingly sending away all your data to a party who promises they will treat it well (I know it doesn't work like this in this case, but just for the sake of argument).
In essence, what you're doing is training an assistant to learn all of your details of your life and habits and the question is if that "assistant" is really secure forever. Taken to the extreme, the assistant becomes a sort of "backup" of yourself. But yeah it's an individual decision with the pro's and con's of this.
At least with gmail and chat clients etc. things are somewhat put in compartments, one of the services might screw up and do something with your emails but your Messenger or WhatsApp chats are not affected by that, or vice versa. But when you bake it into the OS (laptop or phone) you're IMHO taking a much bigger risk, no matter what the intentions are.