But you have to factor in travelling to and from the airports which, for the budget airlines, are not usually the most convenient ones. You also get to see some scenery and maybe even talk to some locals.
For a specific example, I was looking at going from Amsterdam to Berlin (and back). We ended up flying EasyJet from Schiphol to Berlin Schönefeld, both of which simple to get to by public transportation. Total travel time including to and from the airports was something like 4 hours, arriving at 10 a.m. By contrast, to get a train from Amsterdam to Berlin arriving at 10 a.m. would have taken all night and it would have involved several transfers in the middle of the night (at least as far as I could tell from the trip planner we used).
If there were a dedicated express train running at a convenient time, I’d definitely prefer a train trip, though.
My son went to Europe for 2+ weeks back in 2016 - and for him, the Eurail pass would have been a good idea. Bear in mind, a main purpose of his trip was to travel around on the train, and he bounced around several countries (UK, France, Germany, Austra, Hungary, Romania). Would have been even more countries but scheduling issues meant he skipped the Italy / Switzerland / Adriatic route.
For someone hitting only a couple of places, versus travelling every day or two, it’s less of a good deal.
His lesson: you need to purchase that rail pass in advance - there’s apparently no way to walk up and buy one once you’ve arrived.
Financially, it makes a lot less sense than it did back in the 80s or 90s. If you book a specific train two weeks in advance, 500 km trips in Europe are often only €19. Even if you never use two out of every five of those tickets, it still generally beats a pass.
Me and my partner used the tickets summer before last. There are a large selection of them now, such as a limited number of days, limited countries etc. The big advantage is not having to plan ahead, being able to go where you want when you want, and not worry about missed trains.
Our experience was that the ticket works well in the Benelux countries, Germany and the UK. Just get a list of which train companies are not included, its a short list. You can do a lot with just the Intercity trains. I believe Scandinavia and the other Germanics would be good as well.
France, however, requires reservations, often not much cheaper than a ticket would have been, and there were hours in queue for a reservation. I got the impression that France actively tries to discourage Interrailers, they have enough tourists with real money. Iberia works well trainwise, but the topography means the trains take a lot of time to get anywhere, limiting your range.
One other thing to keep in mind is how popular European travel is these days. Even if you could just turn up at a random train station with a pass that could get you anywhere, you’ll have to think about where you’re going to stay. While I’m way past the hostel age, I’ve been looking at my European trip for May and I’ve got to make hotel decisions now. I can’t imagine winging it or go with the flow and know that I’ve got a place to stay.