Earlier today I installed updates across the board: iPad, MBA, Watch, iPhone, AppleTV and HomePods. So far so good.
I did try Universal Control. The one test I ran it worked very well/seamless. And it may be the first time I said "I wish I didn't have the small iPad mini" .
It’s been downloaded and installed on my iPad without issue.
iPadOS 15.4.1 is available. I’m downloading it now.