The main problem I've seen, as someone who commuted daily on one of the busiest motorways in Europe for fifteen years, is that on high days and holidays there were people who didn't realise that cars can accelerate and brake. So on a free-flowing motorway, they aren't able to match their speed even to Lane 1 before they start to run out of sliproad, and they try to slow down to leave the motorway far in advance of when they need to. And if for any reason they have to lift slightly, they're then in far too high a gear to continue to accelerate to either have to change down (and nervous drivers don't change down to accelerate) or continue slowly.
But when more recently I was in the passenger seat permanently in the brace position of a car driven by someone who had recently passed their test, I could see why. In a small-ish car, they were changing up ludicrously early: as soon as they could hear the engine, pretty much, up they shifted, so they were in 5th by the time they were doing about 45. So (a) they spent all their time on the slip road changing up (b) they were using the least effective part of the rev range (c) they were nervous of the car and (d) they had no idea of what the rev counter meant. I drive autos, so don't have this problem, but on the occasions I drive manuals (and I did commute in a 1.0l Metro for a couple of weeks back in the day), I don't redline the car in every gear, but I run it up to the point where it's obviously losing power, which will be probably within 1000rpm of the red line (on a petrol, diesels will vary).
When I used to see nervous drivers crawling down the sliproad, I now know they were probably thrashing with the gear level and clutch to get their small car into fifth by the time they were half way down it, and then wondering why they can't accelerate. It's perfectly safe to run a car up towards the red line (and most cars now have a limiter to stop you going over it, and various mapping hacks to discourage you from getting too close). Accelerating onto a 70mph road in a loaded small car is going to involve using all the power the car has.
The same applies, mutatis mutandis, to slowing down: it's perfectly OK, and indeed safer, to on a free flowing motorway swing onto the slip (assuming you can see to the end of it) at 70mph and then brake. You don't need to be slowing down in Lane 1 to turn off a motorway, in normal conditions.
Accelerate smartly onto the slip. Slow down on the slip coming off. It's OK to rev the engine. It's OK to use the brakes.