In my opinion, it requires a lot of skill to be constantly correct and find the right call over a sustained period of time which Yardeni managed to achieve.
While the right call in the last decade has been constantly to be bullish that does not necessarily mean it was always easy to find that correct bullish call. (E.g. I was very unsure in 2020 after Corona started how equities would perform and partially failed to take advantage of very attractive March 2020 levels)
Any person who has been bearish over this decade missed out completely and any investors who turned partially bearish (like me) missed out in part.
Yardeni is certainly not one of the persons who are blindly bullish. (like e.g. many crypto youtubers)
His S&P predictions are very specific, data-based, and reasonable, and if look up his old comments on Linkedin you see him worrying about FOMC minutes released at the beginning of January 2022 that signal the start of QT. He also states in Jan 2022 that the FED can’t back off as inflation is at 7% and it needs to restore it’s credibility.
After the start of the Ukraine war, he discusses the inflationary effects of it (coming from deglobalization and higher oil prices), shares his prediction of stagflation and consequentially adjusts his S&P price target.
Both predictions came way before we reached the same conclusion and if you read his arguments you will find that they are sophisticated, knowledgable and take a variety of very important considerations into account. (Basically the same factors we are exploring as well)
On the question of how early one should be able to predict that problems are coming, I shared my view about difficulties here. You can see that the S&P only starts to break down one week after the collapse of Lehman as the majority of investors only realized the problem then. So I am not sure if it is feasible to predict it already in April 2008. (Imo we should be careful about the real estate topic as well right now. See: Real Estate - #2 by moritz)
While I do not understand the specifics of the situation in 2001 I think it is a bit difficult to judge someone based on an opinion from 20 years ago.
Additionally, it appears the market hit a local button in Sep 2001 and surged strongly in the next couple of months and has been up for an additional half a year after he made his prediction.
So we would not know if he changed his opinion at one point after Oct 2021 and not even if the decline in Sep 2002 was fundamentally justified.
I also recommend looking up Yardeni.com to form an opinion.
It offers a trove of data that we should leverage more often in our research going forward.