Putting aside limitations, which effectively make it not worth to try iMacros for people who don't have 100$ to throw away, I found a serious problem with memory and processor use in FF 64+. I started to observe serious delays in opening new tabs (when one have many of them eg. 80-100) and switching between them. Using Task, the new feature of FF65, I realized that iMacros use really HUGE amount of processor. This is not related to use of iMacros - the effect was observed without iMarcos active. Everything went to normal when I disabled iMacros...
Hopefully, someone will create something not so blown up and free.