"Another of the home installs on here looks like the flex is connected nearly direct on to the inset stove.
I always put at least 300mm of flue on before the flex"
The stove manufacturer said a 6" flex pipe adapter fitted using fire cement was more than sufficient for the type of stove design, and seeing as i used 904 grade stainless flex liner (rather than the inferior 304 grade) connected to a suitable rain cap n' cowl i am perfectly satisfied that it will not cause any issues.
With my choice of inset stove and bad design of fireplace/chimney it would have been near enough impossible to insert any length of rigid pipe up into the chimney void from the stove exhaust/register plate when fitting the stove due to the angle of the inner concrete chimney breast.
Not every stove install/chimney breast is able to take a straightforward fitting such as you mentioned however that does not necessary mean the subsequent install is compromised or unsafe in use, in an ideal world it would all go together as per the book but sometimes you need to work with what you've got.
I don't burn unseasoned wood, my mate has at least 50tons of seasoned wood (mostly elm and oak) stacked according to year in a barn on his farm taking from windfall in his forests and as i help to cut/collect it/stack it it's all free to me, so tar build up or hot spots is not a problem i need to worry about.