The primary windings are real thin. They can corrode off.They are most all over rated for their use. They overheat and the windings short out then open. Get a higher current one and it will probably last longer.
I was thinking about getting one that lets you switch voltages and has a max output rating of 1.5A
Within reason larger is better. Larger rated unit wastes more power so you don't want to get carried away.Try getting a switching power supply rather then transformer, as there are more efficient./tom