Saya's Vigil being required for Venus slows down the progression from singleplayer into being able to actually play with your friends who are excited to play with you and help you unlock planets
The quest is bugged for new players. When you return from your first bounty in the plains, it doesn't let you talk to Konzu until you reload Cetus
The quest is bugged for old players. The quest forces you to equip a codex scanner, and simaris scanners don't count. If you try to run the mission with just a simaris scanner anyway, it adds a codex scanner to your gear wheel automatically.
The quest contains spoilers. Sentients are a big deal! You're not supposed to know who the Orokin fought in the great war until you reach Uranus and complete Natah. Mentioning sentients that early is fucky. Why explain them to brand new players?
It doesn't help with progression. While the story is fine, and it has some touching moments, it doesn't help the player unlock anything except the codex scanner. It's a lot of time to sink into a pure story quest when the first thing a new player wants is to power up and get out of the Newframe zone of progression and into the good stuff.
It STILL doesn't help make the new player experience less confusing. Yeah, it explains codex scanners, but my friend still needed a lot of help to get it started.
It's too hard to start. You can only start it by doing a level 5-15 bounty in the plains. That's too hard for a brand new player who only just killed Vor!
To add some positivity and contrast to this post, I just played Call of the Tempestarii for the first time and it's SO GOOD. Ghost ship, space shanty, going Moby Dick on a ghost and a burial at sea. It's the perfect story centered Warframe quest.
I was just talking to my friend who played Saya's Vigil right after they made it a requirement to get to Venus, and she's dipped her toes into Duviri, but she says she had no idea that Duviri is full of spoilers. I had to pull a single sentence out of the marketing for Duviri paradox to show her. That sentence is "all Tenno can choose to begin at the ending and play The Duviri Paradox". That's all the warning new players get that Duviri hints at the Operator, straight up spoils the Zariman, lets you do Transference... Duviri isn't a "quick-start" option, Duviri is Warframe 2. A whole other game, that anyone can get and start playing, with its own new player experience, but which assumes every spoiler from the previous is out of the bag and just tells you all of them to get you up to speed.