Well, more of them are in the Fuglsang mould than the Špilak mould, but there are plenty of people who have underachieved in Grand Tours relative to their perceived talent because of inconsistency. That's the key factor really, needing to be consistent over three weeks. I mean, take somebody like Richie Porte. It took him nearly a decade to better his first GT result which was achieved with a time gift from a breakaway, and despite win after win in major one day races, he would always capitulate in the Grand Tours. It's taken him until 35 to finally make a podium. Then there's people like Rui Costa, who have won the Tour de Suisse three times, four podiums at Romandie, one at Paris-Nice and one at the Dauphiné - and he only has one top 20 in any GT, though he has won multiple stages of the Tour including in the midst of week 3. Frank Vandenbroucke won Paris-Nice, the Österreichrundfahrt and the Volta a Galiza (not a high level race but a mountainous one) but apart from his 12th in the '99 Vuelta registered a 50th and three DNFs in Grand Tours. Davide Rebellin has both won and podiumed Paris-Nice, Tirreno-Adriatico (though it wasn't really a GC riders' race when he did in fairness), the mountainous but not high-level Brixia Tour (winning twice) and the Sibiu Tour, podiumed País Vasco and the Ruta del Sol, and managed several top 5s and top 10s at Romandie, Suisse, País Vasco and the Deutschlandtour (at the era when it was targeting being a GC rider's race during the Ullrich era) and apart from two top 10s all the way back in 1996 hasn't been remotely competitive at a Grand Tour, recording 11 DNFs from his last 12 attempts. Marc Soler has time to rectify this, but he has won l'Avenir and Paris-Nice, got two top 5s at Catalunya and a top 10 at Suisse, but has always struggled with up and down form in GTs which is largely why he has never advanced to the stage of justifying leading one. First half of Luís León Sánchez' career is similar, he got things together in 2010 to make the bottom end of the top 10 in the Tour and the Vuelta but never even came close to that again.
Then there are people like Kruijswijk and Enric Más, whose results in GTs are better than over one week because their ability to recover is actually one of their greatest strengths, as they get better as the race goes on, or rather their level deteriorates less with time. Somebody like Špilak, who simply cannot deal with a three week race (and presumably that issue with the heat is a large part of that, since dealing with heat has to be a large factor in two out of the three Grand Tours in the present calendar), is a rarity as most riders can switch their aims around and drop time to stagehunt or go for secondary jerseys, or modify their tactics to suit the team (Rohan Dennis was the strongest climber in week 3 of the Giro, but if he had had to be up there at the front for the first two weeks and hadn't had the chance to drop time to the tune of two hours, would he have had enough left to climb at that level? Impossible to ever know), but the number of riders who have the talent and can feasibly be good after three weeks, but can't do it day in, day out for those three weeks without a bad day that costs them, is many. Plus of course there are people like Porte who do eventually figure it out, but it doesn't come until after their best physical years are behind them. And hell, there are some riders who have an insane palmarès of short stage races and are great enough that they do still score great GC results at Grand Tours, but struggle to convert that into the kind of level of wins that their results elsewhere suggest they should have - Sean Kelly being the ultimate example of this.
However, making any real pronouncements about whether Remco Evenepoel is or isn't such a rider at this point in his career is way, way, way premature.