In short:
Eurail is far more expensive than other forms of travel through both France and Spain due to the additional fees. It is also extremely inconsistent. I know of multiple friends, and myself, who have been completely screwed by last minute cancellations in France. It will be the worst part of your trip and the most expensive by far. Organize your own travel through these countries.
In detail:
Eurail is fine through Germany, Switzerland, and Belgium.I would recommend using it for those countries.
In France and Spain (Spain actually being where I live) it absolutely sucks. The additionally fees to travel through France, on the very few trains that are actually available to eurail customers, can nearly double the price of the trip. In addition, to actually be able to use your eurail pass in France, you need to book months in advance due to them limiting spots for eurail customers (presumably to make more money on full price tickets). This is risky to do because when France goes on strike, as it did without warning the day before my trip began, trains will randomly cancel with no prior notice given by the service. My direct route through France was cancelled, but the first time this information was given was when I arrived at the train station in Barcleona, leaving me stranded in the city. After arguing on the phone for upwards of 40 minutes I was able to reorganize my trip to older train routes that were much slower, and busses. Essentially France was half the cost of my trip, which had 4 other countries and 5 other cities, and was a terrible experience on old trains and busses that I had to fight to have my ticket transferred to (although it was a beautiful countryside to ride through I will admit).
The worst was the experience of my friend. A teenage girl was told “tough-luck” in Lyon at 9 at night when her train was cancelled last minute. She had nowhere to stay as that was only a stop to transfer trains, and had to self organize a metro trip, and overnight bus ride then initiate the refund on her on.
I have friends visiting me in Spain next year who want to travel and I absolutely advised them against using Eurail at least in France and Spain. Not financially ration-able compared to other transports, and not worth it for your blood pressure. I had a female friend ask me if eurail was safe to solo travel, and I warned her that it is absolutely not. There is a high chance that a cancellation will leave her stranded at night in a foreign city with no plans. You are also entirely responsible yourself for arguing over the phone, most likely paying international fees, to get any form of refund or transfer.