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Why do recent UCAV/UAV designs mostly use lambda wings (such as Taranis and Dassault nEUROn) instead of pure delta/diamond wings (such as X-47A Pegasus and Lockheed D-21)?

As for stealth characteristics, both wings have a direct advantage in radar wave reflection.

radar wave reflection for delta and lambda wing
The image was taken from Technology challenges of stealth Unmanned Combat Aerial Vehicles (Sepulveda & Smith, 2017).

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    $\begingroup$ Radar receiver is at where the transmitter is. As long as the wave don't reflects back, nobody cares how the wave scatters from the rear. $\endgroup$
    – user3528438
    Nov 26, 2019 at 18:08
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    $\begingroup$ @user3528438 A radar receiver and transmitter are not necessarily colocated. Welcome to the world of Multi Static Primary Surveillance Radar $\endgroup$
    – DeltaLima ♦
    Nov 26, 2019 at 20:24

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Aspect ratio.

Look at your two images: both wings offer the same wingspan, but the former requires a lot more wing area for it. Wing area is weight. More wingspan for less wing area means better fuel efficiency and thus better range.

Poor aspect ratio, leading to excess induced drag, is a major disadvantage of delta wings. Much of modern stealth design is an effort to mitigate the poor flight characteristics of early "stealth-only" solutions.

Neither UAV nor UCAV get meaningful benefit from supersonic flight, since they can't realistically oppose manned fighters. Large UAV and UCAV are meant for long-range predictable stealth missions, where slow and steady wins the race. Better fuel efficiency means more payload or longer-range missions, both directly improving the vehicle's usefulness.

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