Here's the thing: with more reliable information on gaps(?) where should a solo rider put out the most effort? The answer is simple: on the hardest parts of the course where chasers tend to go into survival mode until they are back into territory where they feel more comfortable. Particularly if the lead gap was made on a hilly terrain. It's the best energy-expenditure plan if you can manage it.
A good soloist will ride a hill below max but hard enough to, theoretically; distance followers but not waste too much energy on downhills. That changes as the solo rider gets closer to the finish, of course. If you are stronger you'll maintain or increase the gap until the followers start to yield to fighting for lesser places. Seems like Tadej does it consistently. Remco can as well.
Here, an isolated example, like Amstel, was used, but he was also failing on the climbs. When you're cooked, you fail on everything; it's not a question of the terrain, it's a question of being cooked.
Lioran's example is a good one. Pogacar lost a lot of time in last climb and the sprint against Vingegaard, something unusual.That's synonymous with being cooked, not that he's suddenly a slow cyclist. Pogacar has beaten Vingegaard in most of sprint, an isolated instance due to being cooked does not make him slower.
Amstel example is an example of being cooked, not an example of him being bad on the flat.
That day, he also lost time on the final climb. In Lioran, he also lost time, and it was the only stage. This is what happens when a rider is cooked, either due to a previous race or poor nutrition; they will fail on any terrain.
But when he´s won 8 Monuments and 2 World Championships, and in none of them has the flat been a problem for them to maintain the lead, I think it's clear that he doesn't have a problem on the flat when it's a road race and on a road bike.
Pogacar's problem is the time trial bike. He hates it and only trains on it before GT races. But this isn't a conclusion; he's always said he doesn't like it.
He tends to use the gravel bike more when he rides with Mathews than the time trial bike.... It's his biggest flaw, and he doesn't seem inclined to improve it much, except when forced before GT races.
But when he takes a road bike, he's not a rider who's ever had a problem riding alone.
And I think that in shorter races, Pogacar is also good on the flat. Always talking about a road race on a road bike.
Honestly, I think Pogacar isn't dropeed by Remco like Madouas and most other cyclists. And not all of them drop out because they're not as good on the flat; it's more a matter of physical exhaustion. Pogacar, riding on Remco's wheel wouldn't have so many problems in a road race.
In fact, it's very significant that, despite he was coocked in Amstel, Pogacar wasn't dropped and was fighting for the win. It's very difficult to drop Pogacar on the flat, not even on one of his worst days of the year did they dropped him.