Many reasons, I may forget to mention some... Over the years vendors have experimented different compression algorithms in order to improve the performance of the product. AOMEI was the fastest in creating full image backups in 2015-2017 for free products just of use PPMD and LZMA methods but failed sometimes to restore from single or from multiple incremental parts. Macrium, on the other hand, has no incremental features and fail sometimes to create bootable media or verify the image. EASEUS just changed the dresses and fixed some cosmetic bugs over the years but I never liked its backup components. Their partition software is superb compared to TodoBackup. EASEUS just don't like Linux GPTs.
Paragon never gave me a single problem... EVER. I don't like its restoration or image checking speed but, in scenarios that I gave it, Paragon just did it. If we talk about reliability we need to talk about product's bootable media. It's paradoxical to expect that program may save your machine, put in the same hell where everything can go wrong ( C Partition). So, my recommendation is to use a program's bootable media. Always. Because of speed and everything else.
Testing reliability is mostly useless (except if done correctly for weeks and months) exercise because some may test software 100 times and it will work ok, others might try it once and it can fail the first time.
Since
@upnorth mentioned it, Acronis has a giveaway link that still works just fine, It may give you the best features/performance/stability ratio.