It will not support Windows x64 for a fact.
I am running x64 - the workaround is to make use of context menu 'Send To" -> FlashRenamer.
The left sidepanel (where one can define what rename actions should be taken) regretfully cannot be resized.
So, if you have a long regex then this is a bit 'so-so'. Also if you have done a number of renames, one can select a previous rename using the drop down menu. However, if they are all 'long renames' and are starting the same way, then it is difficult to see which one you actually need. It would be better to have that panel at the bottom, but for sure that won't happen.
These are the only minor thing I can think of.
As nowadays many such tools are for 'free', users got accustomed to it and do not wish to pay anymore.
Tools like these require quite some development, testing and support and such a fee is really peanuts considering the time spent on it. Personally I have no problem paying for software.
p.s. sometimes renamers have a limit on files to be renamed...
Flash Renamer can handle quite a lot of pages, I mean a lot, 10, 20, 30,000...