The mono is loved because it looks brilliant and is rarely attainable by those who want a replica. The other reason is that it is dynamically superior to a spaceframe if properly designed.
The double D style mono used for the GT40 will produce a much stiffer chassis than all equivalent spaceframes. I see lots of spaceframes with redundant tubing and more importantly, tubing that sustains bending or shear loads.
A pure spaceframe will consist of tubes that only experience tension and compression loads. I've yet to see a GT40 spaceframe that matches that ideal in all areas.
The main issue is stiffness. A chassis that is stiff enough is almost (I did say almost) always strong enough, but a chassis can easily be strong enough but not stiff enough. There's nothing weak about the twin 4" round tube chassis of the Cobra but even FFR add the centre backbone for stiffness.
Essentially, the stiffer the chassis, the more predictable the suspension control and therefore the better the acceleration, handling and braking.
To achieve this, a spaceframe must be fully triangulated and this is the difficult bit. You need "holes" for the driver and all other bits where triangulation gets in the way and when left out, these are weaknesses in the spaceframe.
With a mono, the chassis can be designed with two load bearing structures and a massive hole in the middle (Lotus 23, GT40 and so on) and manage huge torsional stiffness. This gives better suspension control and therefore better handling and hopefully lower lap times.
For the same structural weight, the well designed mono will provide the stiffer structure. For example, the Lotus 24 (spaceframe) of 1962 managed 1000 ft.lb/deg and the Lotus 25 (mono) of the same year managed 2400 ft.lb/deg. The Lotus 25 chassis was 2 lbs lighter than the Lotus 24 (70lb vs 72lbs).
However, spaceframes are easier to design and cheaper to prototype which is why they're so popular for low volume production.
I want a mono but will likely end up with a semi due to the limits of my one-off garage production.