Worst which can happen is a lot of frustration when you try your new coco and find it performs much less than a 1/4 wavelength groundplane (Spider or Cantenna).
No harm in trying. If one is lucky, his coco will perform better than Spider. If unlucky, his frustration will be a lot less if one is aware that the chances of ending up with a poor coco are high.
I don't understand your scepticism. I've made a fair few antennas for ADS-B over the last four yeas since I started dabbling. They have all cost me pennies to make, some worked quite well and I could 'see' planes 200 miles off, some were really poor and couldn't 'see' more than about fifty or sixty miles. The video I referenced covers pretty much all you need to know to make a great success. A pal of mine a hundred miles away also made one - we both found stark improvements over our spiders. You are absolutely correct about getting Velocity Factor right, but this is usually available in the coax specs. And anyway - any antenna dabbler needs to consider velocity factor in putting up an antenna of whatever type, even if it is an HF dipole for the forty metre band. If the wire is covered by insulation, the VF will be different. It is frankly not true to say you NEED complex test equipment. Simple mental arithmetic and cable of a known VF is all that you need besides an accurate ruler and a sharp knife.
The point you made about accurate measurement and assembly applies equally to a 1/4 wave whip. This is especially true at these high UHF frequencies - HF - not so much, because minor errors then become a tiny fraction of a wavelength.
The signal collecting capacity of eight or twelve properly cut, half wave antennas stacked i correct phase is significantly more than a quarter wave - to say otherwise is frankly far from reality.. My collinear is an eight element one. It took me an hour to knock up including climbing about in the loft and poking it out of the roof. The coax was lying around as an off-cut so it cost nothing, the electrician's conduit tube cost £0.99 and is twice as long as it need have been and I am seeing about 1300 to 1500 planes a day at over 250 miles.
I am experimenting with two old net-book computers so I have two feed pages on Flight Aware. The current one is here:https://flightaware.com/adsb/stats/user ... tats-26363
In a few days time from now, my page will be here:https://flightaware.com/adsb/stats/user ... tats-27881
I would also say that this antenna was easier to make than my several spiders since it has no soldered joints. Correctly dimensioned collinear antennas will ALWAYS knock spots off a quarter wave whip.... It is a simple matter of physics. Look in any antenna hand book. The VF of the cable MUST be known or carefully estimated. Mine had a spongy central insulator core so I used a factor of 0.8. If it is hard polythene then use 0.66. The calculation is detailed in the video.