Is Disk Utility reliable?

Yes, it is reliable. Run sudo badblocks -b 512 /dev/sda to scan the disk and find where the bad sector is. It will show the number of the bad sector. You can then verify it with:

sudo dd if=/dev/sda of=/dev/null iflag=direct bs=512 skip=sector_number count=1

Make sure you type the command exactly as stated, using the correct sector_number from badblocks. This should fail with an IO error ( since the sector is bad ). If it doesn't, then you got something wrong. If it does, then you can proceed with trying to repair the bad sector:

sudo dd if=/dev/zero of=/dev/sda oflag=direct bs=512 seek=sector_number count=1

After that, check the SMART status again and the pending_count should have gone down. If the reallocated_count is still zero, then the disk is fine.