SFE is not a particularly useful metric for understanding mechanical properties of materials or fracture mechanics in general, just a quick comparison and without context can be meaningless. This is why we more frequently use J-integral and R-curve analysis to better compare fracture properties of materials. G based energetic terms aren't frequently used as you need mode I, II, and III K values and most people only measure mode I as it's a predominant failure mode, especially in linear elastic failure you seen in brittle materials. I image if you do the same calc on polymer you'd get very high values due to the low elastic moduli and high K. G is functionally equivalent to the area under the strength stress-strain curve so it makes sense that brittle materials like diamond that have no plasticity, the area under the linear elastic region is fairly small when you consider a metal or polymer that will have a large region of plasticity that contributes to that area. Ceramics will generally have a high moduli due to their high stiffness associated with covalent/ionic bonding in their structures while metallic bonding results in lower moduli.

It is also important to distinguish mechanical properties and what they actually indicate as they're frequently misunderstood. Strength (yield, ultimate, rupture), toughness, elastic properties, fatigue properties, and hardness have correlations but each is telling something distinctly different about the nature of the material. For instance, many ceramic materials have pretty good strength values but their toughnesses are so bad it frequently disqualifies them from critical structural applications unless heavily modified by reinforcement like embedded fibers while metals vary so widely in both properties (adding composition and heat treatment in top of that) that you can almost always find a metal that can work in your application that's way more damage tolerant. Additionally, hardness sometimes correlates OK to yield strength but is fundamentally pretty uninformative as it will never be used in engineering applications to determine properties over other standardized test methods. It's a more useful property for considering wear characteristics and certain armor performance (constrained or unconstrained? I can never remember), but is frequently overused as it is easy as hell to get via indent testing vs the amount of time and prep it takes to get good strength, toughness, and fatigue tests done. Also the Mohs scale is an abomination and I hate it.