when tcollect goes (sorta) buggy....

[quinyu]

Perhaps one of the (agreed, very rudimentary) metrics that could be used to test if something is simpler after simplify() is to run a count on the operators (unary as well as binary etc. but don't count parentheses). If the result of simplify() has more operators then it didn't get simpler.

I am sure one could fine-tune this, but that would be much up to personal preferences (I would count a complex number on its own as an addition and a multiplication - addition of the imaginary part to the real part, and the imaginary part containing a multiplication with i... because I don't like to see many i's there, if I can avoid them. But that's just my way.) Either way, I'm certain that simply counting the operators is a relatively straightforward operation for any algebraic expression in most if not all CAS systems.
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For an example I have just found: the derivative of sin(cos(2x-5)). The returned expression is actually the simplest that I know of, on the HP Prime. Clicking [ simplif ] or using tsimplify() however turns it into a considerably more complicated form. Same if you process the expression (cos(5*sin(x/3)))'. Good derivative, then nightmarish "simplify."