ALONG THE LINES OF ANOTHER ANSWER IN CERTAIN CONTEXTS:
As pointed out in another answer to this question, the TLFi/CNRTL sense #4 of dur as an adjective leads to the notion of “looking bad/awful/terrible,” which is certainly appropriate for use in the first, ces souliers-là, example.
Avec ces souliers-là, tu es [pas mal] dur/difficile à regarder!
(Autrement dit, toi et tes souliers-là, vous me faites mal aux yeux!)
In fact, as far as I can tell, this “hard to look at” notion would work well in nearly every context involving literal appearances and/or the particular sense of sight (Definition 4-d) [En parlant de la vue] …), including “en parlant du visage” [still under Def.4-d)] where it captures nicely for me the ingratitude that I see being criticized in the Wiktionary example about the person refusing a free trip to Florida.
(but see sense #5-B-3 as an adjective “[En parlant princ. d'une pers. ou d'un organe sensoriel (gén. dans des loc. fam.)] Qui n'est pas suffisamment sensible” to the extent that this lack of gratitude could also be viewed as a lack of sensitivity)
IN OTHER CONTEXTS
In contexts not clearly involving people/things being “hard to look at,” however, (as is perhaps the case in the second, notre affaire, example), maybe a more general, not-sight-specific insulting notion of someone/thing “being hard to take/support/digest/swallow” (from TLFi/CNRTL’s Def. 3-c) of dur) would be more appropriate:
Franchement, notre affaire est/va être dur à supporter/avaler/digérer.
This basic "hard to take" notion could possibly be extended in some contexts to someone/thing “being hard to take seriously,” and from there, even to simply someone/thing “not being [very] serious,” which I think captures well the idea expressed in example 2:
Franchement, ça [ne] fait pas très sérieux, tout ça/notre affaire.
ET ÇA CRAINT OU ÇA CRAINT PAS?
In Wiktionary’s example about fearing China, the sense of “ça craint” you mention would work especially well (as would “ça va faire peur,” I think), but I'll have to defer to the good discussion you cite regarding the appropriateness of that expression's registre.
In the "soulier" example you specifically ask about, however, I think invoking fear (even sarcastically) with “ça craint” might be overdoing it a bit, and even with Wiktionary's "China example," where conveying/predicting fear is clearly and specifically intended, I think one could still rely on the original notions of "hard to take" (NOT extended to “take seriously”) found in Def. 3-c):
Là, ça va bien, mais le jour où les Chinois vont consommer autant
que nous, ça va être dur à supporter/digérer/avaler.
SLIDING INTO HOME FROM A TOTALLY DIFFERENT ANGLE ...
Another angle that I considered involves comparing “faire dur” with the similarly constructed notion of “faire le//son/ton [petit] dur,” where only an article or possessive determiner distinguish the two.
(see TLFi/CNRTL: "III.− Emploi subst- C.− Subst. masc. et fém.-2. Pop. Personne prête à la bagarre et que rien n'ébranle")
Personally, I usually interpret “faire le/ton [petit] dur” (especially when directed at me!) as a sarcastic way of ridiculing someone by expressing essentially the opposite of what the words could literally mean, i.e.,
“being a tough guy”= “trying (unsuccessfully) to be a tough guy” or “putting on a/your tough-guy act.”
Although I wouldn’t necessarily say that this “tough guy” notion of “le dur” as a noun is the notion of “dur” as an adjective in “faire dur,” I think it is possible that “faire dur” could be intended for use to similarly express, in a sarcastic and ridiculing way, just the opposite of what[ever] “[faire] dur” could literally mean in a particular context, including the “fort” sense of “dur”:
“Tu fais fort-là avec tes souliers…. NOT!”
“Ça fait fort ça, notre affaire….NOT!”
(see/cf: “II.− Emploi adv., fam. A.− Avec force///a) [En parlant d'un cheval] Avoir des réactions fortes/// Soleil dur = Soleil intense, fort,” all again from TLFi/CNRTL)
... WITH NO CONCUSIONS IN SIGHT:
Finally, pursuing this “faire le dur” vs. ”faire dur” angle a bit further led me to the following entry on page 325 of my hard copy of Pierre DesRuisseaux’s Dictionnaire des expressions québécoises (of which, unfortunately, I can find no on-line version):
ROFFE. Etre (faire le, son) roffe (and toffe [angl. «rough and tough», garnement, canaille]); être (faire le) dur [= en France] Etre
(faire le) dur à cuire.
Although this entry essentially just addresses the “tough guy” meaning of “le dur” as a noun in “faire le dur” mentioned above, it was interesting to me that “faire le” is only included parenthetically between “être” and “dur” where it’s used not as a noun but, as in “faire dur,” as an adjective.
I found it interesting as well that one of the words used to explain this “tough-guy,” “être-/faire-le-dur” meaning of “roffe” is “canaille,” which according to my personal hard copy of Le Robert-Micro is “A familiar term of affection applied to/used for children=Petite canaille!” (my translation), just as Wiktionary (transitive verb, sense #3) indicates that “faire dur” is used, albeit rarely, in Quebec.
So with these last few points in mind, maybe the “tough-guy,” “faire-le-dur,” and especially the “être-dur” meaning of “dur” is more closely related to the meaning of “dur” in “faire dur” than I originally thought, which could also justify revisiting the argument (to the extent that "tough guys" are worth fearing) that the full "fear" sense of “ça craint” (still no opinion on its registre) could perhaps fit nicely in all contexts.