For quite a while I dived one AI computer/transmitter, a backup non-AI computer, and a SPG on my left chest D-ring. I ended up using the SPG on a few occasions. I let the battery go dead once on my AI computer. I let the battery on my transmitter go dead once. I had one transmitter hardware failure. In 2019, I switched to two AI computers running off a single transmitter. Once, I forgot to install the transmitter on a reg set. Once, I had loaned reg set due to 1st stage dysfunction. I had an over pressure valve failure on my transmitter and lost both AI computers and used my SPG.
I've been diving an AI computer or two since 2010 along with an SPG. I have used the SPG on 17 out of 1994 dives. Overkill, maybe. I have not missed a single dive or a series of dives in this entire period. Ha, I have an analog compass on my left wrist also though all I usually use is the north pointer on my Teric. Much of my diving is solo, I have no buddy backup for anything.