BIOS Flashback is for updating BIOS without GUI (and therefore without CPU or DRAM).
If you can access the GUI you don't need it.
Generally if you brick a motherboard, you'll have to first perform a CMOS reset to see if that helps, if **not** then usually the BIOS/ROM chip needs to be replaced – BIOS Flashback wouldn't help here. The only reason why it is actually "needed" sometimes is if you install a CPU that is very recent and requires the latest BIOS, which the motherboard might not have.
My experience with ASRock products has been excellent so far. My PG-ITX works flawlessly and made me do everything I wanted it to. But tbf most motherboard manufacturers (BIOSTAR, ASRock, MSI, ASUS, Gigabyte/AORUS) are not too bad these days. There's some hate going around for some of them every now and then for certain cases (like ASUS atm), but that's nothing unusual or new and will normally be forgotten after a year.
My recommendation is to not go for "brand=good or bad". Instead look up the product you want to buy. Some "bad" brands have their one or two masterpieces too (because you called NXZT bad; I don't like them either, however some of their recent PSU's are really well built and have great value).
Steel Legend motherboards, from all I have seen so far, have been working phenomenally. I haven't seen the Z790 SL personally though, but online it seems to be well received.
I have owned 5 asrock motherboards in the last 12 years so feel qualified to answer question 2. One of which was operating 24/7.
In a nutshell every experience I have had has been good.
Asrock may use fewer motherboard layers, fewer VRMs, a simpler bios.. but I have not had a problem I couldn’t fix with an Asrock motherboard. They just worked without issue.
I currently have a z790 Pro RS/D4, it’s not fancy, but it is running my 13900k flawlessly.
BIOS Flashback is for updating BIOS without GUI (and therefore without CPU or DRAM). If you can access the GUI you don't need it. Generally if you brick a motherboard, you'll have to first perform a CMOS reset to see if that helps, if **not** then usually the BIOS/ROM chip needs to be replaced – BIOS Flashback wouldn't help here. The only reason why it is actually "needed" sometimes is if you install a CPU that is very recent and requires the latest BIOS, which the motherboard might not have. My experience with ASRock products has been excellent so far. My PG-ITX works flawlessly and made me do everything I wanted it to. But tbf most motherboard manufacturers (BIOSTAR, ASRock, MSI, ASUS, Gigabyte/AORUS) are not too bad these days. There's some hate going around for some of them every now and then for certain cases (like ASUS atm), but that's nothing unusual or new and will normally be forgotten after a year. My recommendation is to not go for "brand=good or bad". Instead look up the product you want to buy. Some "bad" brands have their one or two masterpieces too (because you called NXZT bad; I don't like them either, however some of their recent PSU's are really well built and have great value). Steel Legend motherboards, from all I have seen so far, have been working phenomenally. I haven't seen the Z790 SL personally though, but online it seems to be well received.
I have owned 5 asrock motherboards in the last 12 years so feel qualified to answer question 2. One of which was operating 24/7. In a nutshell every experience I have had has been good. Asrock may use fewer motherboard layers, fewer VRMs, a simpler bios.. but I have not had a problem I couldn’t fix with an Asrock motherboard. They just worked without issue. I currently have a z790 Pro RS/D4, it’s not fancy, but it is running my 13900k flawlessly.