Hi,
I see 2 problems:
1) It could be a problem if you want to have a home page with links to all instances of your shops.
Like shop selector or linker to the shops, since there seems no option to achieve that
in MULTISTORE FEATURE. If you have completely independent shops , on diffrent URLs,
then this will not be an issue for you.
2) Also if you will use SHOP GROUPS (like: USA + CANADA + MEXICO is Group1 and SPAIN and PORTUGAL shop is Group 2)
so you will be able easily bulk manage the stores in those groups, there will be no SINGLE SIGN ON possibilty for shops in diffrent
groups.
However, problem 1) is bigger for me.
Robert