[One (M9) Q&A, Help & Troubleshooting] Flippy498:
That can happen if you post in the forum of a device that's owned by a mod. ;) Don't forget that we're normal members, as well, if we don't moderate.

If you face a soft-brick you only need to keep both volume buttons and the power button pressed until the phone reboots to its bootloader. From there on you can boot to TWRP and only need to flash a working rom or to restore a backup of a working (stock) rom and the problem should be fixed. Hard-bricks only happen if you flash a CDMA firmware on a GSM device (or vice versa) or a Sprint firmware on a Verizon device (or vice versa). The phone will still be booting but from what I observed here on xda your SIM card won't be detected, anymore, even if you re-flash your phone's original firmware. Aside from that you can "kill" this phone if you're impatient while it's installing an update and power it down during the process (there was a time when that happened quite often). Therefore, I suggest you to just let it do it's thing after you started the update process. Depending on which update you want to install it might take up to approximately half an hour and the phone might reboot (and therefore vibrate) several times.

Correct, for flashing custom recoveries/roms/kernels you don't need S-OFF on this phone (as long as it's not the Verizon variant). Take a look at the further reading section of the ReadMe thread. There's an article linked that explains S-OFF. And Sneakyghost's firmware thread in the development section should contain an explanation, as well. If I remember correctly it should be located in the 5th post. Be aware that you don't need to (re-)lock this phones bootloader like you needed to do on older HTC devices if you want to flash a RUU with S-ON as long as you use the SD card method. That's the recommended method, btw.

I personally don't use a custom kernel. They had a huge impact on my last device (the HTC One S - released in 2012) but HTC seems to be learning at least from some of its "mistakes" since the battery optimizations are much better on the M9. Therefore, I haven't been using a custom kernel since one of the early versions of ElementalX. On the other hand, that means that I can't tell you whether the current version got optimized that much that it actually has an impact, again.

In regard to the nand issue: Don't forget that 95% of the posts here get made by people who face problems with their phones whereas most people don't post if everything is working fine. That might cause that an issue seems to be more common than it actually is. And as said in my last post, I haven't seen such a dead nand in real life, yet, and I know a lot of people who own this phone.

And last but not least a little tip: Find out your phone's firmware as soon as you get it. If it's a pre-4.x version (everything before android n) then try to find the latest 3.x RUU for your SKU*. If you install that one you will save some time that would otherwise be needed for installing a huge amount of OTA updates. (A RUU only needs around 5 minutes if you use the SD card method.) You can't directly install a 4.x RUU since HTC changed the encryption keys between firmware 3.x and 4.x. Therefore, you need to install the 4.x update via the software update function of the phone since OTAs aren't encrypted. More information, some useful files and instructions can be found in the ReadMe thread. (It's a big wall-of-text but reading and understanding it is worth the time that you need for doing so.)

Edit: * = Here's a download link for the latest 3.x AT&T M9 RUU (directly from HTC's server). However, now that I think about it I actually can't tell you whether the AT&T variant already received android n...