What defines a good tonearm

As with most 'purist' audio concepts like 'valves vs SS'....'belt-drive vs DD vs rim-drive'.....'horns vs panels vs dynamic speakers'.....there is no one 'correct' solution or answer.
Rather, it is in the execution of a particular solution whereby a particular design may excel over another?
I have unipivots ( or dual pivots) like the Hadcock GH228, Phantom II and Copperhead.
I also have double gimbal bearing arms like the DaVinci 12" Ref Grandezza, Fidelity Research FR-64s and FR-66s and Micro Seki MA-505s and I have a double knife-edge bearing arm....SAEC WE-308.
You would think that if there were 'differences' inherent in the design philosophies of these arms alone...... they would be audible in side by side comparisons?
Apart from the Phantom II not liking high-compliance MM cartridges and the Hadcock not liking high-energy cartridges like the Titani, I find the differences between the various arms to be those of execution and quality.
There are far greater differences between cartridges than those between differently-principled arms IMHO.

I came to the tentative conclusion that in part I was responding to highly euphonic "imperfections" that I have heard before with unipivots in my own system.

A strange conclusion Lew especially when transposed alongside "the ethereal quality of the Talia" which you heard?
Sounds like you are buying Raul's claim that anything which sounds good must be attributed to 'distortions'?