I'm pretty sure that using an Elixer on an item over +4 is the only way that it can be destroyed....might be +5. "Immortal" on your Item will prevent this from happening.
Any failed Elixer will erase any + and set it back to zero. "Astral" on your item will prevent a full reset.
Other risks come from "Attribute Assimilation" on stones. 10% is the lowest chance that Attribute Assimilation will occur, but special stones automatically have 0% (Astral, Steady). A 30% Attribute Assimilation is the highest chance.
Attribute Assimilation means that theres a % chance that after having a successful alchemy, another stat could be reset. Example - I used a Disipline Stone on my Lv 24 SOS Shield, It worked and I got 9 Critical, but My Block Ratio went from 17 -> 15
Also, you could get the "Maximum durability reduced" where its basically the reverse of a master stone and can not be fixed. "Steady" on your Item will prevent this from happening.