I'd disagree a little bit on the modern finishes since I use them allot on my holsters(though I have also made an absolute ton of traditional finish rigs

The modern stuff depends on what your using, a good acrylic laid thin and proper should flex and hold up pretty well since it more less absorbs into the top layer of the leather. If you lay it too heavily then it will crack and flake there is a fine line though of getting it right and allot of beginning makers run into that issue. Sheaths and holsters share or can share allot but modern holsters can be a whole other critter in design and function. More the rigidity and how it's used into the design is the reason myself and allot of other holster makers don't typically recommend oil on our rigs. Mine are sealed(unless someone requests something else), and such oil and wax won't get much penetration, but they will to some degree. For instance I was working on a shoulder rig this morning and when setting the snap for the thumb break I noticed that the strap that goes around the rear was a bit stiffer and whatnot then I prefer, this is at the end of my process so the rig is already clear coated waxed and edged. I grabbed some Lexol and worked it into the strap and let it sit for a hour, came back and it had absorbed through the clear into the leather(I worked into the back as well but it's also sealed). Then rewaxed, buffed and finished it out with the strap a little more conditioned etc. The point though is with modern boned holsters the retention is based purely on the rigidity of the leather and how it stays rigid and the oils can bring that down to where it's not properly retaining the gun and eventually cause a failure of the design if not the leather. The leather likes to be conditioned, but not too much of course but keeping it right at that line I think is something that is difficult and maybe not conducive to long life leather products. What I mean is that as a holster designer I'm pushing the material to the end of what it's capable of, I have to balance how long it will last vs. how well it will retain the gun since the two are sometimes at odd's with one another.
Anyways point, sheath's depending on the design welted will take a far amount of oil and abuse before they get floppy so to speak, a holster sometimes will not. I've seen some guys do horrible things to leather rigs like dunking them in a bucket of oil and bragging about it(go on youtube there are videos

and I didn't want someone to read this and think that's normal. Best practice is to contact the person who made the item, sheath or holster and follow their instructions on care over time typically they will have seen how their actual process needs to be taken care of and see more than the average person as fair as failures and reasons for them. Some guys finish with traditional methods and some with modern but they are just different and have pluses and minuses. I tend to finish sheaths more often traditionally because I'm building for older knives more heirloom stuff and it tends to fit with the look of the knife more so.
Oh and I forgot to mention above, clear coat finishes, lacquer, acrylic and all the other compositions and brands all do different stuff and handle flex etc differently. I've also found that if acrylic clear coat freezes it will apply just fine, but then fail very quickly after. Once it's applied it doesn't seem to matter, but if it freezes in the gallon or jug you'll find it will fail quickly. One of my suppliers will not ship me clear coat during winter since it can freeze in shipping and then fail. Being I live up in the UP of Michigan that's a good chunk of the year so I have to make sure I've got enough on hand to make it through the winter with my business

If you look up... it was Lou Allessi's business partner or helper oh Skip Ritchie and his early work he shipped to Lighting Arms I believe several years back you will find customer who got a bunch of clear coated rigs that cracked and it was determined his clear coat froze then was used causing them to fail even between his shop and customers in shipping causing a big hoopla. I met Skip out at SHOT a few years back good guy, and talked to him about it heck of a hard way to get a business started
Either way take care!
Luke