I have the U3818DW that I now use with a Mac Pro.
Initially I wanted it to be a hub for my 16" MacBook Pro but the Thunderbolt 3 port is spotty at best, when the MBP is closed the display will connect/disconnect intermittently and eventually you MBP will crash and restart. This is an issue that has been reported by multiple ppl on the Dell Support forums and the answer as always been "We do not support Macs" so they dont put serious effort in fixing that issue. The funny thing is I also had the same issue with a Lenovo laptop...
Anyway all that to say that while its a great monitor if you're planning to use it as a hub - lets say instead of buying a CalDigit TB3+ that I'm using now - I would not count on it, I dont really think Dell cares, they'll tell you it works with any TB3 computer (your Mac) when you're asking their sales and tell you to f*** off when you realise the "hub" part actually doesnt work fully as expected once you have it...
Of course you could hope that the new U40 would fix those issues, but I would be surprised if it was the case. It looks exactly like the U38 down to the ports selection, I would bet it has exactly the same software/everything just with a slightly higher resolution panel.
That being said I'm quite likely to buy one, because they are great panels and the 9W speakers in there are actually quite good - equivalent to a ~50USD speakers you'd put on your desk imo. Also Apple really doesnt do anything nice that isnt the XDR

Just dont buy it if you expect the TB3 & KVM functions to be awesome and flawless...
(I used the KVM part for a bit and while it works, its really cumbersome to switch between the machines and sometime the USB portion will give up and require a turn it on&off again cycle for your USB's to come back)