is a steam ROW key more limited than just a steam key ? ( not activating in US )

SteamGifts rules prevent listing any key that has any region restriction, because there's no way surefire way to make sure people who can't activate the key don't enter.

Weird usually ROW means works anywhere. What game was it?

While most games with ROW would activate everywhere, there are games that have ROW , NA , AU all three being mutually exclusive, because of regional pricing and publishing rights.
Dead island I think is one of those examples.
Tomb raider 2013 on the other hand is labeled as ROW and can be activated both in the US and EU,and I think AU as well.

ROW and WW are not region restricted. Only games, that have region restrictions like NA, RU etc, can have ROW or WW tag. Games that are region free will not have any tag on them.
For example if you see Sleeping Dogs with out any tag, it means that all steam gift copies of the game are activating worldwide. But if you see for example Bioshock ROW it means that there are also Bioshock RU, NA, etc.

The name itself Rest of the World assumes that there is one part of the world where it won't work and another part where it will work. The big question is the proportions between these two parts and that's up to every individual game. Usually the world wide(or you can consider them as All the World) games don't have any tag at all.

You can't actually say "this tag means that".
Basically names are just for convenience and only hint at possible restrictions like low-violence or region lock.

The best way to find it out for a specific key is to head over to steamdb or such, search for the package that fits the name the key activates as and check that one. Retail keys will have "Proof of Prepurchase Only" as billingtype.

Then for activation region locks you'll have PurchaseRestrictedCountries set - if AllowPurchaseFromRestrictedCountries is set it's inclusive, else that list is exclusive. If onlyallowrunincountries is also set than it means there's also a run-time region lock, i.e. you cannot run the game outside of the countries listed in there. For an example of a region lock check for example Dead Island Riptide.

A new type of restriction is low-violence based on country - such packages will have "LowViolenceInRestrictedCountries" set and will be low-violence in the countries listed in ViolenceRestrictedCountries. For an example of that see Counter-Strike Complete (note that this is not a retail key, but it's the only one I know of right now that uses that kind of low-violence).

And finally there's also the simple low-violence versions that don't depend on where the game is run, but only where they key originates from. For those you won't see something on the info page itself, but rather see an extra app listed for the package. E.g. check The Darkness II Limited Edition Retail Germany which has "Darkness II Low Violence" listed in the app list.

In a nutshell: just from what the key tag says you cannot really judge whether there's a region lock, a low-violence version or any other kind of restriction. It all depends on what that tag means to the publisher because they make up those tags and set all the flags.