Is there any drawback in activating memory and cpu hot add on every machine? I tend to think when some handy feature comes disabled by default, it's because there's some negative impact from using it. Sometimes, I'ts saved me from rebooting some critical services that needed their ram to be resized on the go.
vNUMA is disabled if vCPU hotplug is enabled: https://kb.vmware.com/s/article/2040375. This generally only matters for servers with many vCPUs and extremely high RAM usage like a big database server.
Memory hot-add is recommended for any compatible OS: see the compatibility guide here, and make sure you're using the newest OS, VM hardware, and VM Tools versions you can.
Remember that while you can safely add CPU and Memory, you can not remove it without restarting.
i always enable it bij default when creating a new server. The only drawback i can think of is that a vm may crash when adding cpu or memory.