They can be super effective. Or not great.
If you're making progess, running your routine and getting stronger, you are probably better off just lifting. I don't think isometrics will make you lifts go up 50 pounds in 6 weeks or anything.
But if you are stuck, like you can't make progress, they are like magic. If you have a recurring sticking point, or area where you always miss, or some technique flaw, like hitching the dl, isometrics make you better in 2-3 workouts. You start pushing/pulling in the weak position, then "figure out" the strongest body alignment/bracing for that position. Once you improve the position, you start making progress on the regular lift again.
I physically sucked so bad, I couldn't get down to the bar to deadlift. I was sub-beginner. I couldn't get better, because I couldn't even get started. Then I did some isometrics from the lowest position I could. Soon I could reach the bar on the floor. I was super weak overall, but able to get to productive work to start progressing.
Matt Ladewski has an article on EliteFTS about isometrics for deadlift. He said he was stuck around 700 for awhile, maybe years? Then he did isos, and improved his technique off the floor. This allowed him to get rolling again and dl over 800 about two years later.
Matt Wenning said something similar. Stuck around 600, worked his technique with isometrics, got things moving again, then progressed to 800 in 2 years.