If you regularly travel long distance and international by train you'll know that rail reliability is not something you can completely handwave away.
I'm.a complete evangelist for travelling by train. I love it and I'm committed to it as a low carbon form of travel. I've travelled extensively for both work and leisure, putting in full days of travel (just back from Italy this week; have taken toddlers to Prague on the train, etc)
BUT train travel still lags behind air travel in that it's a major pain in the butt to book. High speed trains have a very short booking horizon, and anything with multiple legs will need a heap of contingency. High speed trains sell out rapidly in peak travel times so you can't make plans before you know you can get a reservation. RENFE sometimes don't release tickets until a few weeks before departure.
This makes multi leg itineraries quite fragile and hard to plan. For example, Seat 61 recommends a minimum two hour window to change between Eurostar and another mainline station in Paris and I'd agree with this even though Gare de Lyon is an 8 minute RER ride from GduN and Paris Metro is much more reliable than TfL.
Northern Spain, Netherlands, North East Italy, most of Germany and Switzerland are all beautifully accessible by train. But Scandinavia, Southern Italy,, Greece and the Adriatic and Portugal are all a bit of a mission and labour of love to reach on the train. A holiday that's been planned as flights plus transfer doesn't usually translate well to being an intercity train holiday and it's not helpful to pretend that it would.