I have no experience with Harleys, 2 or 3 wheels (my first bike was an Indian, though, so make allowances for prejudice

), but I will speak to engineering.
Any 3 wheeler, single front, two front or a sidecar rig, is inherently unstable when turning. Note how all trike ATVs are no longer sold; the manufacturers were being sued, left, right and sideways, and losing after the testimony of the plaintiff's M.E. professor expert witness.
The newer Harleys have some stability systems, but the older ones pre-'19, had abs, period. The Spiders have had an elaborate system, involving wheel speed sensors, suspension lift, yaw sensor and acceleration sensors from the beginning. Sometimes Nanny, as the system is semi-affectionately known, is too solicitious, but it's almost impossible to flip a Spyder, something which cannot be said for the Harleys.
From a safety standpoint, especially pre- '22, the Spider wins.