That's perfect for studio work or other work where the photographer can take the time necessary to work that out from shot to shot. For modern photographers, such non-time-constraining situations allow to simply chimp their way to a good exposure in a couple of shots.
Otherwise, not so much, with probably the same or worse failure rate as modern TTL. As an example, that gets especially tricky in run-and-gun situations where each shot counts and there's no time to do a lengthy setup and calculations for each one, where everything can be changing pretty rapidly, from camera distance to subject, to flash(es) distance to subject, to lens focal length, to aperture requirements, to shutter speeds, to backgrounds, and so on. The photographer shooting flash in manual in such situations needs to be able to take all of that into account when setting their flash manually for each shot, on the fly, in order to get "perfect" exposure for each shot. Some few can do that consistently. Most can't.
By contrast, modern TTL does those calculations in a fraction of a heartbeat, measures the actual light before taking the shot, again, in that same fraction of a heartbeat, takes it's educated guess and, 99.9% of the time gets it right. At the very least, it gets you in the ballpark so that IF it's not exposed to 100% satisfaction, it can at least be saved in post processing with a slight exposure adjustment.
Like anything else, you need to learn how to work TTL properly to take best advantage of it. It's not just "any settings, point and shoot, and it magically works perfectly 100% of the time".
Check this out: