Actually an atomosphere-vented boiler has a lot of issues, mainly evaporation of the water, then adding more domestic water, eventually the boiler will become filled with rust, calcium, lime or whatever contaminants are in the domestic water..
And the water will allways have active oxygen in the system, corroding all the iron and steel parts.
A sealed system will not have the evaporation, therefore not contstantly have added contaminants in the make-up water. And the
active oxygen will do it's oxidation 'thing' then it's done, no more fresh oxygen is added..
Sealed is better, but does require expansion tanks, PR valves, and proper design to hold the pressure in the system..