My favorite kind of thread, because mama has THOUGHTS FOR DAYS.
I-scream-a Menzel is number one. As you can tell by that name, It's almost all scream/belt, all the time and not even a good one, it's ALL from her throat, no support from any other body apparatus to take the pressure off her cords, no well thought moments to back off, just all go, go, go. Her defenders drive me batty. As if her vocals aren't inconsistent enough, her acting is also questionable, if not straight up unspecific and nonchalant.
I don't think Sierra's technique itself is bad, just from an artistic perspective it isn't great, but still like her a lot.
Barrett, Jessie, Steve K, Alex, and a few others who might make this list I think are victims of difficult roles in impossibly long runs doing 8 shows a week rather than bad vocal technicians. This is a thing, and a THING that needs heavy considering in the future, and I'll get to that. Alex even said that the way Dewey is written doesn't allow for any rest, and certainly isn't written for the capabilities of the human voice, hence he needed a standby. Jenna is doable but still pretty difficult even for the most steel corded singers, I can't blame Jessie for that...the end of a run with a role like that is pretty much always going to be straining. How Steve managed his Once run at all knowing what I know now, is insane...that poor dude. Ramin...is a strange anomaly to me. His being sharp is usually unnoticed because it's easier to notice flatness than sharpness. Science of sound, y'all. Not that I'm excusing him. But he strangely has cords of steel and does what he does without much issue, and how I have no idea, considering it's no secret he's never had a voice lesson in his life.
Now onto that subject of how roles are vocal written aka vocal pedagogy... I'm HUGE on this. It continues to infuriate me that composers these days have next to nil consideration for/understanding of the human voice and what works for it. ALW can take a hike in this respect. I love the vast major of his shows as brilliant shows, but vocal pedagogist, he is NOT. There is a reason why Evita has a designated alternate. And other roles of his should too by now. I will never excuse Idina for her total lack of technique, but the material she had to work with for her most famous roles had no thought to vocal pedagogy either. Again, even the most steel voiced beltresses have problems with that same material...it's not a lack of good technique, it's a fundamental lack of knowledge of what the human voice can do. It has its limits, regardless of what certain vocal writers might believe. That compiled on an 8 show a week schedule for a long run is asking for problems. Hell, we have past proof of this. And this goes into how and why nobody questions an 8 show a week model. The industry has been so brainwashed into thinking this is a standard to uphold, when it's ruined many a great voice. Singers call out of a show because of vocal issues and get lambasted for it, but a dancer does it and it given all courtesy. This goes into yet another discussion meant for another day, but I'll leave it at this...the voice is a muscle, just like any other part of our body and it needs TLC when injured just as a dance or sports injury would.
As mentioned above, I'm HUGE on vocal pedagogy and could talk about this subject for ddaayysss.