The AH Fox and the Ithaca NID are IMO the easiest to repair if needed,,and that includes auto ejectors.
They rarily have major problems unless owner induced but any part can break or wear out.
Yes they will get loose and so will an LC Smith, Parker, even a Win21. They are just mechanisms and they wear. Not much magic about one bolting system over another. Some are easier to remedy than others. The Win21 is one of the easier, the Parker not so much
Maintainance and general lack of it is the reason most have problems. They're not self cleaning, but you can't seem to convince some of that.
Coil springs in the Fox, Ithaca, Winchester21, and Parker.
Leaf V springs in the LCSmith and the original Lefever.
Once in a while a broken spring in any of them,,but the leaf springs are not subject to a frighteningly higher failure rate as some think.
Coil are easier to get, and flat springs are often expensive and more difficult to find though.
Sometimes you just have to make the flat/V springs and that's a lot of labor time. Then get rewarded with a broken spring on the first test bend and do it all over again.
I've got 2 V mainsprings w/ejector trip extensions on them to make now. One is a German mfg, the other English. At least the good spring in the other side comes in handy as a pattern,,if not a reversed one.
LC Smith breaks firing pins quite often. Don't dry fire your guns is my advise. Replacements are available,,some fitting usually required but not difficult. Make them from round stock is usually easier and faster.
Don't take the LCS action apart unless you are aware of the top lever spring (another V spring and a powerful one) that needs special handling upon reassembly.
Not difficult if you know the process, but I've seen many of the guns damaged and altered in attempts to fight to heavy spring while reassembling.
Stock abutment to frame is minimal at best in the LCS. You need all the positive bedding contact afforded or the wood will start to crack if it hasn't already. Just part of the game.
The LCS single selective trigger can be a repairmans nightmare,,as most any of the older SxS SST's can be (Parker in particular).
A few .000" difference in the tangs position can mean working or not working. That's the easy part to fix. You won't find many around that'll fix SST problems on the older guns. Even the Win21 selective trigger can give you fits. Nothing is fool-proof. Usually is was some fool that got inside and adjusted the mechanism in the first place that causes the problem.
Same with selective ejectors. Parker has probably the most complicated mechanism and parts. It's powered by V springs though the mainsprings in the action are coil. Ithaca, Fox and LCSmith all use coil springs to power their ejectors (called 'kickers' by a lot of double gun people)
Not a lot of parts around though the Parker Reproduction gun parts fit or can be made to fit, but those are tough to get anymore too. Old parts are usually just stripped from worn guns and you get old worn out parts.
Some of the new production CSMCo A.H.Fox production parts are usable in repairing the older original guns,,but not all. There is a scaled difference in the two and I don't know if they even sell parts anymore.
FWIW, the 1990's and later CSMCo production AH Fox guns started with ser# 'F205xxx'. This was a few thousand higher than the reported last 'old' production AH Fox and the 'F' prefix added to the ser# at the demand of the GCA.
The Ithaca NID has separate firing pins in the frame. They hold up well,but can be difficult to remove the small staked in set screw internally if you do have to take one out because of a broken tip.
A rotary locking bolt system, not unlike the LCS and the Fox
The Lefever Nitro Special (Ithaca made) uses the same type of set screw secured firing pins.
Not a bad gun. Kind of like the Stevens 311,,built strong, no beauty contest contestant, little to go wrong.
The A-Grade Nitro Special was frame contoured like the NID and had a better stock deminsion on it. They are well fitting guns as they are.
I upgraded one a while back and ended up with a pretty nice result.
The Western Arms SxS made by Ithaca is very similar to the Nitro Special. I think someone told me the frames were cast iron as opposed to cast steel on the Nitro Spec. Could be,,I've only had a couple of them to work on over the years.
You find them marked w/Montgomery Wards and other names very often. Same as for the Stevens 311, 1500, and other model SxS's.
If it's a double trigger/extractor gun,,I'd choose a Fox, Parker, Ithaca NID, Lefever A Grade Nitro Special in that order.
LC Smith is not one of my favorites.
Given a wider choice,,I take a German, Austrian or well made Belgian SxS from the same period over any of the USA made older guns.
Add the English guns in there too, but they get pricey just because they're Brit.
They're not all gems either!