My question is whether or not the miter lock joint is strong enough or not for this application. I became intrigued with the concept of the miter lock joint recently and purchased the bits from Infinity along with their set up jigs and wanted to try them. It may well be that they are not suitable for where I have been planning on using them in this project and if not I need to change my design, but I had hoped that they would work as they would form a nice clean looking corner joint.
A locking-miter profile falls into the spline joint family of joining techniques. Also in this family are dado's, rabbet (drawer-locking) joints, T&G, floating splines... Long mortise/tenon joints, span over into this category.
Of other joining techniques also mentioned in this thread-- mortise & tenon, biscuit, , dowels, domino, etc... All fall into the same family of mortise and tenon-- of which biscuits, dowels, domino, are all forms of floating tenons.
Historically, a locking miter joints were first used for making columns (making 4 pieces of lumber look like one beam.) So the profile was cut alone with
the direction of the grain. This was a very strong joining technique, meant to last for years of service.
Later this same profile was used for medium sized boxes, such as drawers. Using in this way, it is cut into the end grain. This profile cut in this manner can be used as a corner or as a glue-up profile. It is still a strong joint, gluing end grain fibers to end grain fibers.
This is also what M&T joining does- gluing end-grain fibers together, both methods cut in a manner to extend the gluing surface. As you look at structural factors, the larger the tenon or loose tenon, then the stronger it is-- so a dowel is stronger than a biscuit; an oblong loose tenon (like the brand name domino is), is stronger than a round dowel... and so on.
A true M&T or spline joint is stronger than a floating tenon or floating spline, because there is no interruption of the grain fibers.
Considerations for spline joint joining goes by size of the grain and the cross-sectional thickness. from thin to 5/8", a rabbet joint is stronger than a butt joint, but somewhat too small to attempt a miter lock joint. 3/8" to 7/8" locking miter joints. 5/8" and above usually go back to rabbet joints and other spline joints.
Why the range? Because of expansion/contraction, how things fit together and strength. How things fit together in a chest-- and how things pull apart, a spline joint keeps a bottom from dropping down better than a locking-miter joint. That's just the physics of the forces being applied. A locking miter may look better, It may display more talent in getting it correct and looking right, but if you are asking what is stronger,... in that application for that one piece, that is not joint as strong in that application. (Spline keys and dovetail keys would help make a miter or miter-lock joint oppose those kinds of forces.)
But who the heck cares about if it will hold an elephants weight through a long-mast ship's voyage, eh? Lots of fine furnishing didn't make those kinds of voyages.) A locking miter would look nicer (if you get it right)... just as (to me) dovetails look nicer than box joints. Where I love asymmetric dovetailing in blanket chests <> looking as one piece (a mitered joint, locking or not, not showing any end-grain) is another nice effect, look and feel. Any technique, if done right would be strong enough for a small chest to last years. Besides, that joining technique falls along with the design you had in your head and have already started.
EDIT-- Notice I didn't mention pocket screw joints. I use them. But I think they have their time and place. They are quick and easy. But realistically, they are reinforced butt joints. That is not a criticism or judgement. Just an observation of what they are and where they fall into as a category. Judge for yourself on your own.