I have mandarin Ducks that have laid for the past 2 years in a row, I can share a little bit of what I have seen from them. My ducks have also cracked their eggs before, I can't be 100% sure, but it seems it comes from experience, lack of space in the box, and lack of calcium.
This year they laid eggs in all 4 of my boxes.
The first was a tiny one that I built in the tree of a stump (HUGE MISTAKE). It seemed good, she laid about 6 eggs and started setting on them perfectly, I was really happy that she was setting on them good and none of them were getting cracked, then a few days before they were set to hatch, I went to check on the eggs and realized what a fool I was. The mistake that I made was because of the small size, the duck DID in fact eventually crack one of the eggs, and when it did crack it left a lot of bad bacteria under her. Normally in nature this wouldn't have been a problem because bacteriophages and other viruses that help sterilize would have been present to help deal with this. BUT, there was no airflow into the box, especially when the duck was on them because she took up all of the box. So naturally the bacteria, not having anything to stop it, got into all of the eggs and killed the embryos.
Another mistake I have seen, is that the ducks WILL try to both set and lay in a single box if they don't like the other boxes, this is what happened the first year. I had 0 experience at all with these ducks (or any ducks at all, especially ones that lay in these types of nesting boxes) so I blew it off as I have seen my chickens do this, and it always worked out (big mistake). Similarly to the first one, there was very little room for landing, so they broke the eggs quite often, there was a cracked egg that they pushed out every few days laying on the ground. But in addition to the little room there was, the ducks would compete for the eggs, treating them a bit too aggressively, which wouldn't be too huge of a problem if they were chicken eggs, but these are mandarin duck eggs, they are extremely temperamental, even if the eggs are treated perfectly, in captivity, or in an incubator it is very unlikely for all, if not the majority for the eggs to hatch. Not understanding fully how temperamental they were is what lead for me to only hatch 4 out of 20 or so eggs from the first year (all 4 of which hatched from 6 of them that I put into my incubator). And the biggest caution of them all, is the temperature outside, and the time of year, HEAT WILL OBLITERATE THEIR ODDS FOR HATCHING THEIR EGGS. You HAVE to make sure that it stays cool in the box (Not under the duck of course).
And the final mistake that I have been realizing much more recently is the bedding I used in the nester boxes, that played a massive role in all of the other mistakes I made as well. so the mistake I made, was using dried grass and dried plant material that I could find for the nesting material, the problem wasn't in the grass itself, but the fact the ducks found it edible. As you would expect, they got hungry while sitting on their eggs and were like "Oh hey, breakfast in bed" and eventually ate all of their nesting material, leaving only their feathers. Obviously this resulted in the eggs eventually ending up sitting on the base of the box, on the wood, there was no insulation, nothing to pad the eggs when the duck jumped in the box, and this is when they started getting cracked.
This year from what I learned last year I have hatched 12 so far (still 2 ducks setting on some eggs, idk if they will hatch tbh) 8 of which I hatched in my incubator, and 4 under one of the ducks.
I still have a LOT of stuff to learn, but I did learn most of it from their most recent breeding season in the past 2 months. But I have a bit of stuff I can help you with.
If you plan on hatching the eggs in your incubator you will have a MUCH Higher hatch rate if you let a broody hen set on the eggs for the first 10 days or so, when you do get them, keep the humidity High, as the time under the chicken has already shrunk the air cell all that they need (most of the time) I keep my humidity at 65% until day 25, and I mist them once a day with water that has a very small amount of hydrogen peroxide, they do usually hatch at day 30-32 for me, and from pip to zip it is usually a couple days, Lockdown humidity to around 75%.
If you plan on letting the ducks set on the eggs, use material that they won't eat, make sure they have lots of room in the box, make sure air can flow into the box, and don't let them try hatching any more than 8-10 eggs, as the ducks are very small and can't really fit many under them, as well, if they have access to swimming water, MAKE SURE IT IS ALWAYS CLEAN, you don't want the duck to get into really dirty water with lots of duck feces and other stuff, then climb on her eggs.
I do not have much experience and I am always looking to learn more, but I sure hope that I helped a bit.
Also, this is my first time trying to compile this many words into a single post so its probably VERY Unorganized, sorry about that
TLDR; Just read it I can't summarize it, or I don't want to
This is their first year laying eggs though (both are almost 2 years old) so idk. We feed them an allflock feed from our local mill. Maybe adding some extra calcium to their diet wouldn't hurt but they don't seem thin or extra fragile.
But the 2 remaining are doing absolutely wonderful. 
