ICS wants to set the machine its running on as a dns server and gateway for the systems connecting to it. If your network already has this configuration going out through DHCP or some other means there could be some confusion. ICS was really created for small personal networks. Not for larger scale networks that have DHCP and DNS servers.
If you choose to use ICS it will automatically assign IP adresses starting in the low range in the 192.168.0.0/24. One interface will receive a public IP address from your Isp and the other Interface a private ip 192.168.0.1 .If you have another dhcp server or static configured IP addresses you also could have IP conflics. Also if you have other gateways or routers you will not beable to use them. ICS is restricted to that one IP address range and everythign points to the ICS server. So if you need to get to other private networks you will have issues. It may be a choice If you just have a modem and no router and you need a small amount of computers to connect to the internet.