I'm a middle of the road kinda guy.
I tend to believe that every tool can have its place in a program at one point. It's just a matter of understanding what a specific tool can bring to the table.
The coaches who deem single leg work "useless" tend to be those who believe that strength is the only important thing. I value strength over most qualities, but it's not all there is. For example very few people can do only the big basic lifts and have no lagging muscle groups. Most people who only do the big lifts all the time will eventually develop imbalances.
Is single leg work a good way to build maximum strength? I do not believe it is. Sure you can overload a specific muscle or group of muscles, but the overall loading on the body is not high enough to develop overall strength maximally. Is isolation work a good way to train for strength? I don't believe it is. I know plenty of people who can use a lot more weight than me on triceps extensions, pec deck and front DB raise but I can outbench them by 80lbs.
However these two tools can be useful to fix a weak area or to prevent an unbalanced development.
The big lifts will always be my main movements. And I tend to prefer bilateral exercises for my main assistance work. BUT if the issue to fix requires, or is more effectively fixed with unilateral movements, I will use them.
Most of my crossfit athletes do Bulgarian split squats at least once a week. It is often part of their "activation" program to prepare for their workout.