Welcome! 
Of the ones you listed, HomeKit is actually the best for privacy right now, because everything is local, except voice control, and that’s anonymized. But people have all different reasons for choosing different platforms and many of us use more than one, so you’ll have a lot of choices.
As to how easy it is to move from one platform to another, it’s never easy, but it’s often doable. Right now there is a brand new industry standard, matter, which is just starting to ramp up. Its main purpose is to make it easier to use a device with all of the different major platforms. So if you’re just starting now and you want to only get matter certified devices, you’ll have quite an easy time moving to a different platform if you make that decision in the future.
Consumer reports has a good explainer on the new matter standard:
Matter Smart Home Standard FAQ - Consumer Reports
As to the question of whether you’ll have problems with SmartThings, the answer is most assuredly, yes. But you’ll have some problems with any of the platforms, that’s the nature of the industry right now. As to whether the problems are only occasional annoyances or dealbreakers, that depends on your own needs and preferences.
For myself, I started my own Home Automation journey back in 2013, so 10 years ago. After a few years went by, I made the budgeting decision that I would assume that any device I got, including the hub, would have to provide its value in three years, because after three years, I might want to change it. Even change platforms altogether. Maybe it wasn’t what I really liked, maybe there were other newer features that I had decided were important, who knows. But I budget time and money for Home Automation in a similar way to the way I budget my mobile phone. I don’t expect I’m going to have the same one in 10 years, or even five. so I get my value out of it, and I spend my money based on that assumption, and then, if I do change, I’m ready for it. And if it ends up being something that I stick with for longer than three years, that’s gravy, and means extra money in the Home Automation budget to spend on other fun stuff.
If you like long, detailed discussions, the following is the list of questions I usually ask when I’m helping someone get started. But if you find it boring or tedious, don’t worry about it, there are lots of other ways to make the decision, and matter is making it easier these days.