I'm the developer of iCab Mobile, so I guess I can help understanding the issues....
The web engine of the iOS does not support file uploads, but we are not allowed to use our own web engine or modify the existing one. Therefore to support file uploads in iCab Mobile, iCab has to slightly modify the web ages to be able to install some hooks in it, so iCab gets called when the user wants to select a file an when he submits the form and s hen able to intercept the web engine. This is relatively complicated.
This works great without any problem on all(!) web pages which just use standard HTML. It also works great with more user friendly Javascript-uploaders which can do the upload in the background etc. So there's no problem at all on Flickr.com, tumblr, Facebook and many other popular sites.
But there are problems if web pages try to check certain properties of the selected files before uploading (like size, type) because this is simply not working on the iOS platform. Also some web pages break down because of of the required modifications within the web form iCab. But this is nothing that can be avoided right now. These are all limitations of he iOS.
And there are web sites which use Drag &Drop for uploading files. These pages expect the user to drag a file from the Mac finder or windows explorer into the bowser window to upload this file. This is obviously completely incompatible to mobile devices.
So basically, the main problem is when web pages try to check properties of the selected files and refuse to start the upload if they fail to get any results here. Most other pages just work fine.