on rare occasion I have seen an expansion tank leak or rust a hole
out the side of it..
it can happen , but only a few times in my experience...
when they do leak it usually can be catastropic,
pissing water everywhere
I honestly dont care for them but I have one in my own home... its just something else that can go wrong. and over time they can get very heavy with water..
no one maintains them, no one checks the air pressure ,
and no one wants me to come back out
and pay me $150 to change it out 5 years from now either...
its best to install the expansion tank over the
top of the water heater, and mount it so it wont wobble or
topple over some day... I have actually used some heavy guage
copper wire and secured it to the overhead rafters...
if you were to just hang it in mid air and
it breaks off the 3/4 copper pipe due to
a weight issue 5 years from now,
you are soooooo very deeply , totally screwed......