I've gone back and forth between failure and RIR training. First learned about RIR back around 09 from a guy on Getbig under the handle JPM. It wasn't called RIR then. It was just stopping a few reps shy of failure. I find both methods - failure and RIR - to be effective. But I will say that "failure" has some downsides/negatives. (1) My strength, motivation, energy, and enjoyment of the gym all go down. With RIR training I am much stronger, happier, more motivated, etc. (2) It's impossible to reach true failure, which makes "failure" training more relative than absolute, and your less likely to have consistency. (3) Failure training can be riskier for injury. (4) Failure training burns less calories.