Here is how I evaluate assistance work.
Does this add something to your overall athleticism and health?
Does this help improve your main lifts?
Is it safe to perform, either in general or for the person in specific?
If the answer these is yes, great, I include the assistance work. A lot of them fall down
hard on 2 and 3.
An example is rack pulls (a shortened deadlift). I find most of my clients have a problem not with lockout strength but with getting the weight off the floor in good form (or at all). Rack pulls help with the top of the deadlift, which isn't where they are failing, so I usually skip these and work on other things.
Another example are Pallof presses. I'm pretty well convinced these are very valuable for most of the population, both athletes and non-athletes. They are safe to perform, and while they may not clearly improve the "big lifts" of the trainee, they correlate pretty well to improved health, strength, and stability. So I include them. It doesn't worry me that an increased Pallof press doesn't seem to correlate with a higher deadlift or squat or more pullups. But as it goes up, other benefits accrue . . . so I find a way to rotate them in.