1. Wrong understanding. ACS does what you tell it to do. It can assign a guest vlan to unknown assets or assign whatever else you like.
What do you mean with an internet access requiring authentication ? If you think about a guest portal, i.e. a web page asking to enter credentials, then ACS doesn't do that, so you have to couple ACS with a Guest Server. That's a bit of a pity because the Guest Server is not a product that will evolve. ISE just do everything in 1.
2. ACS is not a captive portal correct. When you say "it provides a webauth feature that is fallback", it's wrong. ACS doesn't provide anything like that. The switches implement web authentication (or the wireless controllers) and ACS can authenticate the people using that, but ACS is just a radius server saying "yes/no" and giving privileges.
3.ACS has no limitations to support wireless in particular. What the paper says is that ISE provides a captive portal that will be the same for wired or wireless users.
Remember that with ACS, you need to use the captive portal of the switch and WLC or a nac guest server. So not unified.