I started experimenting with isometrics because my calves do not seem to respond to exercise. Isometrics turns out to be one of those useful ideas, like sprinting, ice baths, and other things highly useful but probably only done if you've got a coach making you do it.
But isometric exercises make the gym machines somewhat useful. I can see some improvement, and recently Ben Greenfield brought up something new to me on Dave Asprey's show. Apparently, lactic acid can be turned into glucose and used as fuel; isometrics increases lactic acid, which in turn encourages the body to upregulate the enzymes necessary to convert lactic acid.
Isometrics also start out faster than doing normal reps and sets, but you get more time under tension with isometrics. You can reliably get forty to sixty seconds of time under tension at whatever given weight and then increase weight. You can also increase the time interval as well, but I suspect there will eventually be a point where there are diminishing returns. There are benefits to full range of motion exercises too, so if the interval gets to be too long, I will probably want to return to something more traditional.
I can see one situation in which isometrics ought to help a lot, though, and that is at franchise and/or hotel gyms that aren't set up for barbells and free weights. One could conceivably manage a decent workout in under 15 minutes.