The only tangible warning sign I personally see at the moment is the "time factor".
Not that I feel the development is going slow, but that Shenmue 3 would need more than 2.5 years (from scratch) to really allow Suzuki-San to deliver what he has he mind.
That said, I'm prepared to welcome a 6 month delay, if necessary.
I'm prepared for a year delay. I just think they don't have enough time / budget to go that long to deliver what I think he will deliver.
I would like to say Shenmue 3 is different given Yu Suzuki's experience, but really scope and time are such big issues. I think when we are shown some real combat which is one of the cornerstones of Shenmue, then I will be able to make a better assessment.
Recently I played my first Kickstarter backed game which was backed in 2013, Hyper Light Drifter. Alex Preston had never made a game before, but he had on his team a number of semi experienced indie developers and also his unique vision and art work. The game was delayed for 2 years given Preston's health problems, but you know what, he delivered a stellar debut of a game. Hyper Light Drifter is one of my favourite games of this year and sits along side Dark Souls 3 comfortable. So while there have been failed expectations in Mighty No. 9 and No Man's Sky, it's not all failures.
I don't think and I do hope, Suzuki's experience will help put them put out a solid game. It's again with the assistance and persistence of Sony Third Party productions with Gio Corsi helping this along, so we'll see how strict they are with deadlines. So far Shenmue 3 still looks to be in very early days of development. The amount of time that planning is a part of a game's development can't be understated and even with the last Shenmue 3 update, Suzuki brought in Okayasu Keiji as sub-director to help with day to day affairs on top of his other production assistants. So really it's to ensure work gets done according to Suzuki's vision.
One other factor is Suzuki isn't doing what Inafune did, which was start the project, leave it and run 2-3 other larger projects at the same time. Suzuki also isn't jumping on the hype train like Sean Murray is, thankfully and cranking up expectations way above what they could possible deliver. Suzuki has been quite clear about what he wants to achieve as set out by some of the Kickstarter goals. Of course some of those KS goals were rather vague, but if you understand Shenmue then you have a decent idea what those goals are.
For me the key elements of Shenmue are 1. Story/Script 2. Combat 3. Models/Animation 4. Environment 5. Mini-games. It's gotta be fun, gotta be engaging, gotta look good.
With UE4 most of 2 - 5 can be taken care of, but story, it has to be right.. we've waited so long to see the story continue, so number 1 is really the highest priority.