LOL I have enemies try to escape, but often they are pursued, caught, surrender, etc.
Relief is rarely felt, since if the enemies are fleeing most often the PCs have the encounter well-in-hand already.
Oh, on that point I argee. Creatures should have more varied speeds, for example. And after the manticore failed escape scene, I have doubted the fly speed for any creature that has a natural fly speed. The speeds listed would barely be fast enough to keep most birds aloft, let alone flying at their typical speeds.
Even treating it as an abstraction, there are some issues IMO that pop up, but general DMs just have to go with it and whatever makes most sense for the scene works for me.
And that is the crux of the issue for most groups.
A hell hound has a speed of 50. So, a PC engaged with a hellhound runs, prokoving an OA, but survives and moves & Dashes 60 feet. Meanwhile the hellhound moves 50 feet, and breathes with a 15' cone, hitting the PC. The PC lives and runs again, the hellhound moves again and either breathes again (if it recharged), or dashes to get in front of the PC and engage them again. Now, the PC provokes another OA, and it continues. The PC also has to worry about exhaustion because of dashing, where the hellhound doesn't have to dash every round to keep up.
You are better of standing and fighting or surrendering.
Without magic to enable your escape, even cunning action won't likely help until you can break away enough to find someplace to hide and manage to beat the DC 20 passive perception for the hellhound (due to keen senses).
It is very hard. But then again, if you ever played tag, usually someone does get tagged. Escape isn't as easy as some people like to think IMO...