I've used UE at previous jobs, and while I'm not gonna say it is the best ever engine, allowing this piece of software in the hands of indies for 19 bucks a month is a bargain. I will probably not use it as I left the games industry, but compared to other commercial offerings out there, the 19 bucks a month plus the 5% is a bargain if any team is doing high end graphics games. You can bitch about Linux or try to do all the math you want to say Epic are being money grabbers, but seriously, unless you had access to full source code before and see what the engine can offer you, please take a moment to step back and appreciate what they are doing here.
ps: not affiliated with Epic at all. Just from many years in the games industry, Unreal has been the best engine I worked with and this is a FANTASTIC offer