Wasn't exFAT designed purely with flash/external memory in mind? At least that's how I remember it.
Yes, that is true. However, considering the extremely large filesystem size and file size limits on ex-FAT, I would not say it was designed only for external media.
evidently, if recent file systems were as unreliable as they say, i would've had much trouble by now but i have yet to stumble upon heavy errors during copy-paste sequence, TeraCopy also helped in this reliable file transfers.
You're not really getting the point. No one said any filesystems were unreliable. Any unreliable filesystem would inevitably cease to exist. The benefits of advanced filesystems are, as said already, protection against soft errors. Copying and pasting, no matter how much data you are transferring, has an extremely low chance of such errors, and any errors encountered would be single bits at a time, which most copy-paste operations could fix with a simple CRC check (note that this is completely irrelevant to the filesystem).
You can get write errors from all kinds of file operations. One that Phlebas put as an example is a "half-assed file transaction from a frozen program or PC." Imagine that a program has opened a file stream for writing and ended up hanging partway through a write operation. Then, perhaps the program is terminated by the user before it is able to close that file stream. That half-written data sits there, and conventional filesystems like FAT or NTFS don't do anything about it. They might not even let you access the data, since it's write-locked to a file stream that never closed.
Other situations can happen as well. For example, imagine that a disk's cache has some dirty pages that have yet to be flushed to disk, or something is currently being written to the disk or cache. Suddenly, the power goes out. What happens to that data?
Also, keep in mind that data does degrade over time. No media can keep its data in perfect form forever. That's what data checks and repair are for.
Overall, I still think most modern file systems and OS will handle this decently, though 
Keyword "modern." NTFS was created in 1993 (most recent revision in 2001), and FAT32, the newest version of FAT before ex-FAT (ignoring FATX), was released in 1996. The oldest version of FAT, FAT12, was released in 1980.