I don't think the trial bosses are too hard.
Whenever a mechanic is first introduced, it's usually introduced by itself so you can see how it works and so getting hit by it will at worst give you a vuln stack and a bit of damage.
The first trial boss is even generous enough to disable controls and zoom the camera out so you can understand how his gimmick works.

If all else fails, a new player also has the option of just following someone who looks like they know what they're doing (or the person with the least vuln stacks).

I have never experienced any of these horror stories people keep talking about in which it takes them multiple groups to get past a single trial.
The only way I can see that happening is if there are several people in the group who refuse to put in any effort to pay attention to what is happening to learn from it -or- there are people trolling to get the group to disband hoping they get something easier in their trial roulette.