Just like any other consumer item, "reliability" is governed primarily by design, build quality and quality control of production.
Products are designed for specific kinds of use. For example, a power hand drill designed for full time professional contractors is going to be designed differently than one intended for occasional home owner use;
- The build quality of the professional-use drill is going to use heavier, more durable parts (metal gears instead of plastic) and those parts will be housed in a more robust case to hold the parts in proper alignment during hard use
- The pro-use drill will also have stricter QC measures in place during production and post-production inspection to ensure reliability
- Of course, the price point between the two will be different as well
So how about flashlights?
The same applies: those designed for hard use,
especially those used in life-threatening situations such as fire fighting and LE, are going to be designed and constructed differently than those intended
for hobby/enthusiast expectations and those intended for more casual use.
Design and build quality are observable. Once way for a consumer to observe these qualities is to disassemble the item and take a look. Here is the tale of two lights;
A Foursevens Quark on the left, a Malkoff VME/MDC on the right. The Quark weighs 2.4 ounces, the Malkoff 4.0 ounces (both with battery inserted).
The Quark disassembled as far as I care to without risk of damaging it (I still carry this light in the glove box of my car). I can observe the following;
- The circuit board is "tacky-glued" to a plastic disc in the bezel and be easily lifted off
- There is little-to-no heat sinking, the aluminum reflector has minimal contact with the PC board
- Not seen in the photo, the flip side of the PC board (with contact spring) is exposed on the battery side
- Overall construction is very light and feels more fragile
The Malkoff disassembled as far as it can be (other than the McGizmo clicky switch and carry clip);
[*=left]The "drop-in" is a single piece with a heavy brass housing that encases the PC and reflector
[*=left]The PC is potted inside the brass housing for shock resistance and thermal transfer to the brass casing (heat sink)
[*=left]The PC is fully protected on the battery side
[*=left]Overall construction is heavier and more robust
Another view;
If the light is dropped bezel first, the cell will impact the potting, not the PC board. The larger the cell, the more inertia it will have when it impacts. Without a mechanical stop in the body, an 18650 cell slamming into an unprotected PC board will result in damage to the PC board pretty quickly.
So in terms of flashlight reliability, with a little disassembly one can usually observe how the light was designed and constructed. Quality of construction is generally a good indicator of how well the light will hold up over time to being left on and getting really hot,
repeatedly dropped on hard surfaces, being subjected to the shock of using the light as an impact tool, exposure to heat, cold and moisture, etc. I would also expect that lights with more moving parts (multiple switches, flood-to-spot adjustable bevels, pivoting heads, etc.) and complexity of operation are going to be more prone to failure than those with fewer parts and simpler user interfaces.
I have not had the opportunity to inspect the lights mentioned in the OP, but my guess is that they're all pretty similar based on their advertised specs and feature sets.
If ruggedness and long-term reliability were my ultimate concerns, I'd be more inclined to limit my list to lights with known robust build quality, such as Elzetta, Malkoff and HDS.
Just my 2-cents and FWIW...
