I've used a TON of DL stuff over the years, as well as a lot of all the different AB and Siemens generations, and a smattering of Generous Electric, Omron and so on, and in my experience, the ADC stuff is actually the BEST for failure rate. The AB and Siemens stuff is good, too, but probably a bit higher OOB failure rate than ADC, especially AB.
I did have one horrible project on an early Control Logix in like 1999-2000, probably around v2.5 with a CPU, a DeviceNet card, the DOS PC module with the RJ45 serial port, an Ethernet card, a lot of analog, three servo cards running 1336-FORCE drives with induction motors and encoders, and miscellaneous discrete. EVERYTHING in that app failed. They replaced the CPU, both comm cards, the analog, the servos, and I think even the PC card, rack and power supply. I think only the digital escaped unscathed.
I also had one site with an S7-300 where a bunch of stuff died in 07 or so about a year after commissioning, but a lot of stuff from a lot of different brands died simultaneously, so it had to have been some electrical issue rather than anything to do with Siemens per se.