I guess it's because they are comparing the service of Stadia with other platforms and of course they figure out that for example Destiny 2 is working much better (latencywise) on console/PC than on Stadia.
And then also a lot of bad reviews were regarding the bad launch regarding the founder's editions, which I totally agree was not the best start for the platform.
But I totally agree with you: For me Stadia works very well. I mean they promised you are able to just login and play without any long downloads, etc. and this is true. I can just press the "S" on my controller and start playing without any problems.
The rest we'll have to see, I mean there are definetely some issues regarding some games that needs to be fixed and also some features that would be very nice to add to Stadia as well as offering more games on Stadia than just 22.
So I would not say, the YouTubers are totally wrong but I still think, some like to make more money and complaining is watched by a lot more people than saying good things about something. This is the internet. 
