The main principles doesn't change whether the design is simple or complex. It is only that in complex design there is more calculation to be made so you require a computer. But that does not in any way mean the basics working principles of creeping wave and diffraction change.This is the reason why a very strong mathematical background required for any complicated engineering design.

This is the same case like in the Quantum chromodynamics and the Quantum electrodynamics .

The centre of gravity / lift can be calculated with the general knowledge of the shape, after this calculation the centre of gravity / lift will not move too much if you consider the exacts shape of edges, holes protrusions and so on.

With the interaction of the electromagnetic field with the body of the aircraft the mathematical method and model is different,

If you add a small detail to the model then the backscatter to the emitter will change not by the relative magnitude of the change, like in the case of centre of lift, but by the same magnitude like with the first calculation .

See ?

So , all simplified model will be useless for the design.

If you add a small objects/detail with RCS of X to a model with a RCS of Y, the total value will not be equal X+Y, because we have to consider the effect of retro directive reflectors, as well as discontinuities, return. But that does not in any way mean the principles change.

It is the same with real life experiment. The main difference between a simple model and a full-scale airplane is the fact that a full scale airplane have many small details and panel gaps. So, generally speaking, there are more discontinuities and scattered locations. But main principles remain the same, return in Mie region is ALWAYS and I really mean ALWAYS frequency dependence. Because in Mie region the creeping wave return curve around the object and interfered with the specular return. The end result depend on whether this interference is constructive or destructive. Whether the interference is instructive or destructive depend on whether the creeping return and specular return are in phase or out of phase. That why the wavelength is important in this region.Again, check the results of real life experiments.

This is a bit more complex than this simplified model.

Just because an aircraft has RAM applied on it doesn't mean it will have low RCS or will become a low observable aircraft. US also applied Iron ball RAM on F-15, but that doesn't mean F-15 will suddenly have the same RCS as F-16, let alone something on F-35 level. Reducing RCS has other benefits apart from making your aircraft hard to detect, putting RAM on your aircraft can help reduce its return and making jamming more efficient. This is the case for F-15 and Su-34. They are not stealthy in any way, shape, or form. Su-34's X-band scatter is not slightly bigger than F-35, unless we consider a dozen thousands times bigger is slight. At most, Su-34 is the same as F-15 with iron ball RAM. Hoping that it can reach the level of dedicated stealth aircraft is simply wishful thinking with no basis on reality. FYI, US does refuse military hardware when their performance doesn't reach important requirements, just look at how many program they canceled.The Russians did a deep and expensive work with the robot applied lamination of the air intakes and so on.

So , two possible motivation :

1. Russians are stupid, and did a pointless work

2. The X band backscatter satisfy the military requirements.

Considering that the Russians has no issue to refuse the delivery of a military hardware it the performance of it doesn't hit the requirements (it is the opposite of the USA practice) we have to consider that maybe the explanation #2 is true.

Of course it gives only frontal low observation characteristic, and it can be slightly bigger in X band than the F-35, but the Su-34 has to penetrate a patriot battery, not an S-300, and even a flying pig with a lipstick can do that : P