actual limitations of locked bootloader I have read in many posts that a locked bootloader prevents from flashing custom ROMs, because it checks the digital signature of the ROM itself.

I thought a "bootloader" was only responsible for loading an image at startup, thus a locked one would refuse to load an unsigned image.

Therefore, wouldn't it be more accurate to say that a locked bootloader refuses to load/boot an unsigned image, but it cannot do anything to prevent one from flashing that image ?

Of course flashing something one cannot boot does not make any sense, but that is not the point of my question. I just want to learn exactly what the locked bootloader on a phone does.
Your statement is correct. :) The bootloader can't prevent images from being flashed, it just prevents unsigned images from loading. My answer to this question has some detail on the way locked/unlocked bootloaders work.

For example, if we consider "SP Flash Tool", an application that communicate with the bootloader of MediaTek devices to flash images, the lock status of the bootloader does not affect ability to flash. (It is still possible to block flashing by only allowing signed binaries to communicate with the bootloader) But still, if an unsigned image is flashed, the bootloader will refuse to load the image.

Still, usually images are flashed using Fastboot (even when there are more options to flash like "SP Flash Tool") and quite a lot of devices only have Fastboot as a known flashing method. Some common exceptions include Samsung, and MediaTek devices. And Fastboot checks most of the time if the bootloader is unlocked, and only lets flashing images if it is. (Same goes for Odin, the flashing application for Samsung devices. It only lets flashing when bootloader is unlocked) So, unless there is a secondary way to flash, it usually is not possible to flash with a bootloader unlocked.