They seem to be broadbanded, small, and ground-independent relative to other cheap wire antennas, so what's the catch?
- Perform poorly for multi-band use, so you need a separate antenna for each band you plan on using.
- Require significant height for their benefits to surpass the typical dipole.
- Due to the shape and height the supporting structure of the delta loop is more complicated than other antenna types.
These can be a small price to pay for the benefits offered by the Delta loop, but they can present significant hurdles for casual amateur radio operators.
Delta loops are popular on the low bands (7MHz and below) because they are much easier to set up mechanically than a quad, but they have a smaller internal area than a quad for the same frequency which slightly reduces their gain in comparison.
They have a higher effective mean height when the triangle is pointing "downwards", but this requires two points of support and is thus less common.
They are strongly dependant on their height above the ground, and work best at the same height as a dipole - half a wavelength above the ground.
Therefore the catch is that they are difficult to erect at a height which optimises their performance.