Personally I would never be a fan of a junction box (I mean a bog standard junction box) in a wall as such.
Whether or not it complies is another issue. A MF Junction box could be made not comply a bog standard one less so.
If I am aware of any junction box then if I consider it suitably accessible I have no problem with that , providing there is a reasonable chance I would be aware of it.
How accessible it needs to be to be considered accessible then opinions vary and that is way I stated "reasonably accessible" considering what I believe to be the intention of regs writers and the opinions of learned bodies.
If it is underfloor etc then it starts to become inaccessible or less accessible to be considered.
If most people are unaware of its existence then, to my mind, it fits a similar consideration.
If it is reasonably accessible providing that you know its there but if you are less likely to know it is there I would tend to give it very similar considerations as a part of good practice.
Perhaps the most important reg in the whole book is one that has remained pretty much the same for years. A simple concept that good workmanship and proper materials should be used throughout.
Whilst any JB buried in any wall may start to give rise to concerns, one that is buried in a wall where the likelihood of damp or wet conditions is more prevalent heightens these concerns or tends to.
If some socket/switch etc becomes, effectively, a junction box (example - a spur on a ring, or a ceiling rose) we are more likely to know where it is and it is more likely we can examine it from time to time anyway.
Probably each one of us has an opinion that we ourselves are a "reasonable person with reasonable thoughts" and what we consider easy is easy and what we consider difficult is difficult but there is no dividing line between the two, there is a least 50 shades of grey between them.