Is there any reason why this SD-Card-Reader would not be compatible with Iphones and Ipads besides the obvious physical USB-A-connection issue …
Just because a product doesn’t mention that it is compatible with ios / iPadOS devices, it doesn’t necessarily mean that it is incompatible. If the product follows the correct specifications (for e.g. USB, file format, etc.) it can often work with these devices too. The reason for not mentioning it could be as obvious as the USB connector used. While you have found a convoluted way to connect the device to your iPad, someone who isn’t as technically savvy as you will obviously be disappointed (and feel cheated) if they buy the device because it mentions it is iPad compatible, and then find they can’t connect it directly to their iPad.
Sometimes manufacturers also don’t mention compatibility as they may not want to deal handling customer support from the users of a particular device and / or invest more money – dealing with someone like Apple, buying new equipments, training, testing etc. all means increased cost.
For example, I am using a generic Chinese mouse that only mentions that it works fine with Windows. But I am able to use it without any issues on Windows, Mac and Linux.
With your setup, the only thing I’d be worried about is whether all the connections and power supplied to multiple devices may cause a problem in one or more of the connected devices.
You have a powered USB hub. That is good as it means it won’t draw power from the device connected to it (in your case the Apple Camera Adapter). And if the USB hub is a good quality product, it will follow all the USB specifications and won’t draw to much power or supply too much power to damage the devices connected to it. Your Apple Camera Adapter is also powered separately with an Apple power supply adapter. This too ensures that the Apple Camera Adapter will not unnecessarily draw any power from the iPad and work optimally.
All in all, I don’t think you have anything to worry about (except the burden of lugging around all these multiple devices with your iPad).