This subject generates a lot of discussion and no small amount of debate, even among experienced instructors.
There's a reason for that ...
Situations and circumstances can vary quite a bit in the real world. Different techniques may prove to be a bit more beneficial in different circumstances.
While debating the relative advantages/disadvantages of the various light techniques and equipment, I'd offer that safety considerations must remain paramount and foremost in folk's minds, during training & in actual situations.
Training to do something with one hand while holding a drawn weapon in the other hand does present some serious safety considerations. I think this needs to be emphasized and kept uppermost in everyone's mind while engaged in training & practice.
Hand confusion and sympathetic contraction can still present potential problems, even when something doesn't occur which startles the person using the weapon & light. Balance destabilization (tripping, stumbling, unexpectedly shifting footing, etc.) can also create the potential for sympathetic clenching. Imagine how trying to hang onto the light, clenching it and functioning the button/switch, might result in a hand confusion situation which unintentionally functions the trigger.
Naturally, when you consider the potential for something 'startling' someone operating within a stressful situation, the potential for a ND may increase. Trigger safety/finger placement is critical.
Yeah, I know. Old hat. Obvious. Everyone already knows this, right?
I can think of a couple of instances where LE involved in a some separate special enforcement activities experienced ND's while holding a weapon in one hand and doing something else with the other hand. One of them was manipulating a flashlight at the time.
I know both cops personally, and both of them have received a level of firearms training unlikely to be equaled by most 'line level' cops throughout their careers.
Once safety considerations are emphasized so they become ingrained and 'refreshed' during training ... then there's the matter of choosing a flashlight techniques ... or techniques ... to serve your needs in all of the situations someone anticipates may be encountered.
I've always used variations of the Harries, FBI and neck or shoulder techniques. Negotiating different things while searching, performing suddenly- required, unplanned different actions with a non-gun hand (being able to contain, reposition or temporarily stash the light source) can create some stress on the user. Remember you may have gun in one hand, too, right?.
My assignment requires that I enter residences and commercial buildings virtually everyday, a number of which are lacking electricity or maybe just lights or functioning light fixtures. I carry a selection of available flashlights in my gear, although I commonly use one of a couple of small units which I carry on my person.
Different buttons/switches can affect things to a degree. So can the diameter, length, overall size and lumen output of the light source.
Larger spaces can require more power, as can the level of potential 'light effect' desired 'out front' of the user. Depends on how clearly you want to see something, as well as the nature of reflective, angled or obstructive surfaces/spaces present. Lots of variables can make for unexpected results. Unexpected splash-back can be annoying, distracting and possibly even dangerous.
Mostly I just want to be able to see, recognize and illuminate potential threats while performing my job. I don't plan on directing traffic during my job anymore.
I have slightly different needs when selecting a light source for off-duty usage. Some obvious overlap, but not completely.
I've been using flashlights as part of my job for the better part of 3 decades. I've found some techniques and methods which didn't work in some actual situations nearly as well as I'd expected. Changes and refinements, in both equipment and techniques resulted. Still working on it.
Adding the safe deployment and use of weapons to those circumstances escalates potential safety issues tremendously.
Attaching a light source to a weapon comes with its own advantages and disadvantages, too, not the least of which is I've encountered a lot of things which I did NOT want to cover with a weapon's muzzle.
You can't call a bullet back.
Keep studying the increasingly available material regarding the safe and effective deployment and use of flashlights/light courses, and consider looking for some reputable, quality training offered for this subject.
I don't claim to have the answer, or answers, but I'm seen some of the involved problems which I'd like to continue trying to address and resolve, if only for my own circumstances.
Fortunately ... for all of us ... the increasing number of good quality light devices available for use, as well as the proliferation and use of them in LE & Military fields, is resulting in some increased understanding of the needs of the users when the equipment is used in potentially dangerous situations.