In that regard, Herbert Pohlmann's "Professor Junkers nannte es 'Die Fliege'" is quite interesting.
Pohlmann (of Stuka designer fame) describes the exponential growth of design complexity over the years, starting with the very simple F13 airliner and carrying through to the postwar concepts he was involved in.
Everyone will agree there.
This growth sort of invalidated everyone's experiences on how long programs took, and in combination with the RLM's expressed policy running flight testing and production setup in parallel (which didn't work) lead to a lot of planning failures. The Lusser-Lahs letter recently discussed on Calum Douglas' site covers this, and Eisenlohr's book adds the information that the development time required for new engines for systematically underestimated, too.
The more modern and the more complicated design, the more the time it will require? Also, not every company was the same. We can compare how long it took for the P-38 to come around, as well as how much it took for the P-47 to became an actually useful combat aircraft that is not trying to kill it's pilot, vs. how fast were the P-51 or the F6F developed.
In the post-war time of 1950s, McDonnel was churning prototypes in an amazing pace, while some other companies did not.
As for the engine development, sometimes the favored engines get the resources, while other are starved for the resources. See the HeS engines. BMW was fiddling with the BMW 802, 803, 804, while the 801E was all but abandoned because there was supposedly no machine tools available. It also took the Germans eternity to come out with the more advanced superchargers for the existing engines.
Germany has the Jumo 004 to install in an 1-engined fighter if they wanted to do so.
With regard to the He 162, Messerschmitt basically stated that the mass employment of the type in the spring of 1945 was an impossible goal, and history seems to have proven him right.
Lack of success of the He 162 had a lot to do with the circumstances Germany found themselves in in 1944-45, rather than with the failings of the design.
When it is about own and other people's products, I've ceased to believe the statements of the people having the horse in the race years ago. That sleeve valves are oh so good. That other people are guilty that DB 605A does not work, but not DB, not us in Stuttgart. That Fw 190 was not a 'big engine on a small airframe', while Spitfire was; that Fw 190 was armed to the teeth from day one, but Spitfire was weak in that regard. That carburetors are oh so great (of cource they are, since you dont have any other fuel distribution system).