NW Vars - What's so bad about them?

Where exactly does the overhead come from when using NW Vars?

I assumed that they only really caused a problems when you have a lot of them being updated frequently on high population servers. For example if you’re using them to store players money on a 120 slot server I can imagine having to constantly update that value on all clients each time it changes would have a fair amount of overhead since the value likely changes frequently for each player.

Is there any noticeable overhead for retrieving the value of an NW Var? So if you were to use them to store a value that never changes or very rarely changes, would there be a noticeable performance impact?

I know compared to DTVars, you have the overhead of networking the string initially for the stringtable, but I’m not sure why NW2/DT vars are comparatively better in repetitive usage.

Either way, you should only use them for entity values that need to be synced often and in prediction. Otherwise, use normal networking.