I suppose a lot of what is being said here applies to reducing signature from the side and top profile. It must be said that the main threat against the missile would be towards the front aspect. So the measures are facing the law of diminishing returns here because they are not applied where the main threat is most likely to be, which is the front.
The main cross section of any missile is owned up by a radome. Today's radomes, made of carbon composite, are also radio selective. Which means they only let the frequency used by the missile to get through. Any other frequency is absorbed. This kills a few birds with one stone, as it improves the signal gain of the missile's radar, blocks off noise and ECM interference, and at the same time, reduce the missile's radar cross section. Any conventional cruise missile or AshM can be regarded to have a radar cross section > then its physical diameter.
Thermally an AshM is also hard to catch because it flies low over the water, and water vapor would seriously attenuate certain IR wave bands. And besides, the main heat source is on the rear end.
The reason I don't like using a flying wing is that it takes up too much space on an launching aircraft or ship. You want the missile to be as compact as possible so your platform can carry more of them. A flying wing missile would also have some very serious flight consequences on the aircraft that is carrying them.
There are low RCS missile designs like the German Taurus, but some of the benefits are still arguable. Having a squared off and angular design like that can mean increased drag, which can have penalties on the missile's range and flight performance, and not just on the missile but also on the plane carrying them. Round and cylindrical forms are against low RCS, but these forms also have less "wetted" area compared to angular or squared forms, which means better streamlining and less aerodynamic drag. These rounded and cylindrical form also offers the lightest structural weight.
When it comes to missile design, the following principles must apply.
Keep it simple.
Keep it small.
Keep it light.
Keep it cheap.
Simple---the simpler it is, the less likely a malfunction would happen.
Small---the smaller, the more missiles you can carry. Small and light has its disadvantage when it comes to range, but for any given range requirement, its best to achieve that with the most optimally smallest and lightest sized package.
Light---the lighter missile can mean less mass, which means more range for any given size. It means a more agile missile as well, for evasive purposes or catching up with an evading target. A lighter missile means more can be carried by the platform.
Cheap---Because it's meant to be expendable.
All these factors serve to align and reinforce each other.