Hi, yes you can use in the Netherlands all trains of NS and the other participating companies at Eurail. You need a valid Eurail Pass with using a travel day and you have to add the journey to the app or writing it on your Paper Pass. 

Also, when reading some of the posts, there were comments that the pass may not be the cheapest route if you have your itinerary set.  Can you explain this reasoning? 

Sometimes you can use 1 day tickets or like on planes if you book soon you will get tickets for lower price, so it could be in some situations cheaper to buy a point to point ticket for all the journey or one leg instead to use the Eurail Pass. Keep in mind that some trains require a reservation to enter the train. In the Netherlands only Eurostar and Thalys high speed trains have compulsory reservations, ICE International and IC Berlin to Germany have only optional reservation. 

Most people use passes with passdays-so its daft easy to calculate what 1 passday cost. Mostly for normal adults this is about 30-35€/2nd.  De dayreturn in NL-where I now sit in an NS train-for a short distance cost considerably less. But f.e. this €* charges 30 extra-making it like 65-and even there a normal advance single may be lower in price-if one only does that trip in that day.

IN all main cities the major means of transit will be the local city-system: metro/tram/bus, these never take passes, but most will offer their own 1day or 24 hr pass. However-mostly in DE and also in a few specific large cities there is a kind of local train-transit too, often named S-bahn, and run by that railway-so accepting passes. But again: vs. passday a local ticket will cost much less. But if you have the continu pass anyway-like I do now for 3 month (bought in the 50% discount offer-makes it about 5€/day) it is included anyway

It might be useful to add that inner city transport such as buses, trams and metros are not included in our pass network. The only notable exception to this is the S-bahn in Germany and certain cities in Austria and Switzerland.