A big killer in radiant floor systems is not enough constant flow while the boiler elements are on.
Like was mentioned spalling of the elements is common and in particularly hard water this is multiplied. filtration of replacement water is very important.
you haven't mentioned the size of your re-circulation pump or how many total feet of tubing you have embedded in your floor or the square footage you are warming.
A friend of mine has a radiant system throughout his house with 3 different types of boilers #1 the main boiler is diesel fired boiler # 2 is built into his fireplace system which is a Russian chimney design boiler # 3 is a 100 gallon electric
there are 7 different zones each are valved and allow constant bypass flow when a zone is tuned to a lower setting or turned off. each boiler has its own circulation pump as well
His system was first switched on in 1998 except for the new digital switching controls the system is still original
He attributed the key factor to having used triple distilled water when installed and if any replacement water added it to was distilled