just my two cents here - surely the issue with trials is how they are run?
No system is ever going to 100% get the best players picked - so trials if run well are a good way to get a fairly accurate idea of your talent in a short space of time. Trials run poorly of course are not ideal.
Trials in my experience rely on two things. A general outline of how players are looked at, IE what skills should be looked for and secondly you need to have a good amount of spotters and ideally the coach of the team is not the only spotter - IE if you are trialing for an entire age group (say 14th grade) that the 14th grade coach is not your only spotter and he is not the person with overall selection calls otherwise bias comes into it.
I really think long observation of a player in a normal game environment is far far better than a trial - so the top team coach should be observing the reserves/ B team whenever possible, and this informs next year's selection. A trial period is OK. A single or several day trial is really only useful to sort out players who are either clearly good enough or clearly not good enough for the team being selected.
I know many B team footballers who've stood out in their grade year after year and have eventually gone on to represent their region.
Mostly, the spotters (or assessors) have to be good! Rather one good spotter than one good and two dodgy ones, muddying the waters. 'Good' is subjective though and it's often the loud, strong and wrong brigade who have the most say.