I disagree with both statements.
"Not spending," AKA trying to get by without talent, isn't a good idea for any part of your team, but when it's the part of your team responsible for run prevention the last 4 innings of every game it's particularly damaging.
I also disagree that relievers are more "volatile" than other players. Good relievers are better than bad relievers. Maybe in the 60 IP typical sample size, at first glance it doesn't look that way, but that isn't volatility. That's random small sample variance and not something to count on.