For not showing, single - I have snaps on the hold back straps (where they hook to the breeching - the hold backs can stay on the cart), snaps on lines at the bit (standard on lower end harness - but I'm slowly getting lines changed out - as, though it's quick & EZ, I've had them snap themselves to objects and each other - could be serious wreck and NOT FUN or EZ to get loose); snap on backstrap (just makes it easier to change out to smaller one if necessary or when I made my harness and didn't have more than one crupper/backstrap for a while, it allowed EZ changes) - buckles for actual adjustment; on braided harness, I have rings & conway buckles.
I have one set of traces w/ snaps on them instead of a conway buckle - I DO like that, but in a hard pulling situation, the snap could break (I've had these traces since 1997 but they aren't used everyday either)... This pic doesn't show it well, but there is a snap on the breast collar end of the trace (military nylon strapping) as well as on the driving lines (lounge lines made from MCR & snaps).
These are actually two lounge lines with snaps - used as lounge lines, too, so won't be getting snaps removed though they can be (put on by a loop - love my homemade equipment - have had a lot over the years).
For not showing, pairs, work harness - snaps are standard on a lot of this type harness (even higher end) and so are Conway buckles. Other parts (on my lower end harness) have snaps that I'm swapping out for sewn in buckles (lines). in pics below, snaps are on the breast strap - both ends - different types. Come standard w/ the harness. The small one down between his legs hooks to a neck yoke.
Sorry this one so big, but shows better too - the snap hanging down from the back pad runs up between the hames & hooks to that ring on the side cheeks (mid-neck). The snaps under his belly are the quarter straps, standard on most work harness - size/weight determined by size of horse/harness. The flat back faces up to the belly as it will sometimes come into contact w/ the belly.
Snaps are nice - make changes fast and simple. But they are usually the weakest part of your harness and it only takes a split second for them to get them caught on something they shouldn't - especially bit snaps!! I've learned the hard way... I've had my daughter stop a riding pony next to a wire fence - he dropped his head down next to it and the snap hooked to the wire. Thankfully, though green, he'd learned enough to give to pressure at the bit and after raising his head, lowered again and we immediately got it unhooked; more than once, I've had my pairs hook themselves to each other when they try to rub their heads against one another or the one that was constantly throwing her head; the shocker - when one of my pairs put her head down while trotting down the road and hooked a bit snap to the ring on the neck yoke - that was a nightmare!! to get loose and she did have damage to her mouth - bruising, abrading & a cut to her tongue (no driving for a while and then was bit shy for a long time). I can probably think of a couple of other incidents - 1 of these happened between 2005-2007 & the other two driving ones between 2011 & 2013 - when I was driving new pairs heavily with bit snaps.
Straps I DO LIKE SNAPS ON -
My connection straps when driving pairs - sure they can break, but so much easier than a buckle. You can see both in this picture (orange braided haystring straps at front connected to hame rings and at back hooked to breeching rings). I would not want to stand and buckle these type of straps, LOL.
Lots of snaps in evidence here, LOL - probably won't get these snaps replaced on the lines and all the snaps are different on the connecting straps (made from braided haystring blue & orange) - here the connecting straps are hooked to their halters not the hame rings. They can pull on each other's heads (especially when that nervous bay spot threw her head all the time), but this was the first time they'd been ground driven as a 4 abreast, so I wanted them hooked at their heads.
and once again, the forum is not allowing me to use the Google Photos... I'll figure out something else... I'm so totally POd w/ GOOGLE right now...