They're total POS.
However, one main problem is due to common user error:
If you close the action with the firing pins in the unretracted position, the lower firing pin will hit the bottom of the monoblock, as there's no built-in purpose-designed mechanism to prevent this from happening - as with many other more expensive O/U's. Over time, not only does the lower edge of the monoblock get dented-up, but the constant assault on the firing pin will eventually lead to the pin(s) breaking. Also, constant denting of the lower edge of the monoblock can affect the action itself - not good.
This issue affects many O/U's, not just this cheap-a$$ Mossberg model, and the solution is the same...
A simple solution is to carefully, and ever so slightly, file the lower edge of the monoblock. Now, if you close the action with the firing pins in the unretracted position, which is easy to do, the firing pin will slip past the monoblock and simply stop on the lower edge of the cartridge and only dent the casing -- no big deal for a $1 cartridge. Of course, if you kung-fu grip it and slam the action shut with too much force, you risk a slam fire, though I have not heard this happen before.
The other main problem with this model is the quality of the firing pins: cheap-a$$ steel. After a while, they will break with normal use.
I understand that Mossberg has dealt with these issues with their newer batch, though I can't be certain.
Overall, however, the quality of this model is sorely lacking, and personally I wouldn't spend a dime on one. Save your coin and buy a better quality O/U, or spend the same amount on a nice, mildly used, but far better quality O/U.