I have a reasonable level of experience of flying in mountains (well high ground!) in IMC. The point is at what height does the IMC start?
It is entirely possible to fly across high ground VMC below a solid ceiling. It needs to be high enough and you need to really, really know where you are. But it is very commonly carried out.
The 'received wisdom' seems to be if you enter IMC you climb and get above safety altitude. Well nice idea, but reallt difficult to achieve.
If you end up in a 'dead end', the chances of outclimbing the terrain are pretty poor. Most light singles struggle to get 1000fpm - as a gradient it might suit railways, to outclimb the average hill it is completely inadequate.
If you run out of VFR conditions you have a very short time to decide whether you throw the dice and hope you are flying up the valley whilst you climb or you force land.
A forced landing up a hill can result in a ground speed of virtually nothing. fly up the slope, hold off and then land - the ground seped if carried out well is virtually nothing.
You do need practice at landing on steep gradients - but once you have that you will neverr be able to understand why airfields are on level ground. The gradient makes an enourmous difference - you can literally park on a hillside. Getting the aircraft will need a helicopter but you will step out and walk to a phone (so long as you have sensible footwear and clothing!).
It would be great to simly fly over the top - but my aircraft does not have the ceiling or icing protection to allow that. I can fly through the valleys - it needs care and occasionally needs the odd U turn. Going IMC, when a precise heading is necessary - probalby with a turn, is all that stops contact with cumulo-granite is not a smart move, I'll land!