Can ext4 fs be completely unrecoverable broken due to power loss while disk is writing?

Suppose you are doing a full speed write to disk in a linux pc box with ssd or mechanical disk (OS also on the same disk, there is no battery/UPS):

cat /dev/urandom > omg.txt

If the power losses suddenly during the process, or any other sort of ungracefully shutdown/reset.

Will the file be corrupted and unable to fix (i.e. none of any data can be recovered?), will there be a chance the file system be completely unable to boot?

Answer

Will the file be corrupted and unable to fix (i.e. none of any data can be recovered?)

Potentially, yes. There's 2 obvious routes via which this could happen.

Ext4 is a metadata journaling filesystem - it only journals the changes to the file's meta data (size, location, dates) - not the file contents (btrfs and zfs do full-data journalling at a big performance cost). So although you should never have to fsck the disk, it doesn't follow that every write operation betwen opening the file and closing + flushing the buffers will have completed. There is no transactional control over writes to the file data.

A second possibility is that the disk may be physically damaged by power spikes. Although the rest of the hardware tends to do a good job of isolating the hard disk, there will still be some leakage.

will there be a chance the file system be completely unable to boot?

That's a very different question - this is a lot less likely. Certainly the first scenario only applies if you happen to be writing the kernel, bootloader, ramdisk etc at the time of the outage.

See also this Q&A on unix.stackexchange