VxRail offers a single package for everything: server+storage+support+vmware licenses. On top of that it adds some additional life cycle management, like updating of firmware etc. I have many customers who use VxRail. But I also have many customers who simply pick a VSAN Ready Node from the HCL (vmwa.re/vsanhcl) and go from there. Either of the two options work. If you are comfortable with updating firmware etc yourself through the available interfaces Dell provices you could go with Ready Nodes.
When it comes to NIC teaming, you can use LACP if desired, or you can use any of the other options that vSphere provides. I typically don't see this being an issue to be honest for customers. With 10GbE and 25GbE, bandwidth is usually not the biggest problem.
To answer your questions:
My opinion is that VxRail is a white elephant, full of things that in fact the final purpose is vSan only.
VxRail is a highly automated platform for setting up and expanding vSAN Clusters. It's basically "power it on, enter data like hostnames and ip-adresses" and the Automation Layer does the rest.
VxRail also offers several other DELL-EMC products that are helpfull in certain use-cases. But the main driver is total automation, easy of management (updates incl. firmware, drivers etc. truly are just a couple of mouse clicks and it actually works).
If I choose vSan, what care do I need to have to choose my servers?
If you purchase VxRail, everything like ensuring compatibility is done for you. If you go the DIY route, then select so called vSAN Ready Nodes from your preferred hardware vendor. This ensures that you don't run into compatibility issues during setup or later on during daily operations.
One factor that made me curious is that VxRail has some specific network requirements, vmNICs amount to guarantee throughput. They do not dedicate vmNICs to vMotion, Mgmt, and so on.
They do actually. In order to make the extreme automation of VxRail possible, the networking is mostly pre-defined. So the networks for vSAN, Mngt, vMotion are all kind of hard coded. One wins total easy of use but one loses the flexibility to "do whatever you want".
This you gain back when going with vSAN Ready Nodes although you must always adhere to the vSAN Networking Design Guide and it's supported Topologies. vSAN uses the network at it's core and it better be damn well designed. vSAN is no toy.