In a low-light situation with a split-second to focus and capture the moment, the flashgun's AF illuminator was essential. Mirrorless cameras usually do not offer you this feature and also their AF systems aren't so great in low light that they don't need it.
Mirrorless cameras have captured up with DSLRs in virtually every measurable respect, however there's still 1 feature that's critical for some professionals that each one of these lacks: flash AF lighting.
Wedding and event photographers in particular will recognize that the benefit of the AF assist goal proposed by flashguns plus a few flash commander units, yet no mirrorless camera manufacturer implements this attribute, even on their highest-end versions. There are some technical reasons why they omit this attribute, but that doesn't negate the need to it.
It won't impact everyone's day-to-day photography, however, the near-universal failure to offer you some sort of flash AF illuminator target for mirrorless cameras stays a major shortcoming and means mirrorless can not compete with the top DSLRs in some shooting conditions.
You will find specialized justifications for why they do not: DSLR autofocus sensors can be sensitive to ultraviolet light mild, whereas mirrorless cameras are not. Since mirrorless cameras concentrate using their principal imaging sensor, any IR has to be filtered-out to prevent skewing the colors within your images. However, the AF illuminators on flashguns are not only emitting infra-red: they also emit lots of visible red light, otherwise you wouldn't be able to observe the telltale red grid that they endeavor.
Another drawback is that phase-detection pixels reside behind the colored filters which allow most cameras to interpret colour into the scene, so may not even see any red light. A PDAF part behind a blue filter will not see any red light plus one behind a green filter is only going to find a very small volume. But this should not be insurmountable: Canon's Double Pixel AF means every pixel (such as the red-sensitive ones) is a PDAF pixel and, when it came down to this the manufacturers could offer versions of the flashguns that exude a color that their mirrorless AF systems may see.
Most mirrorless cameras possess their very own built-in AF illuminators but they're not quite as successful as the off-board lamps on flashguns. In addition, they tend to be bright, uniform orange or green lamps which are visually distracting, readily blocked by palms or huge lenses and that overlook a few of the essential attributes of off-board illuminators: a grid pattern. This grid layout effectively generates a few hard-edges for the AF system to snack on, even if you're shooting something that might not have sufficient inherent contrast to easily focus on. You know, just like a white wedding gown in reduced light.
If mirrorless cameras will revive DSLRs for wedding and event photography, producers will need to discover a way, even if it involves selling (or modifying) flash components that emit a faint green or blue grid pattern. Or doing exactly what Godox does with its X1T control units: firing its red target, just as it might for DSLRs, because in many situations this is far better than nothing.