I've worked on very many different efi systems from the early days.
haltech, accel dfi, felpro, electromotive tec 1 and 2. I worked on a motec equiped turbo v6 in the days of the mustang vs buick era. I loved the motec software. Electromotive came out with "wintec" software, which was a scaled back motec rip off. It made the Electromotive system light years better.(original software was DOS command based) I stuck with the electromotive system for my own stuff. It did what I needed it to do (most of the time) The more modern systems (big stuff, efi technologies, holley, aem) are more laid out like a computer template program. I like the old "cartoonish" icon based software.
A telling thing to me is I have never had to work on a modern FAST system. as popular as they are, I would imagine that either they are very straight forward and easy to get correct intuitively, or their support is outstanding and rock solid.
I do a lot of driveabilty clean up for guys that take their cars to dynos to be "tuned". Getting the high power numbers seems to be the goal at most shops. Driveability is the part that makes guys happy.
I worked on my first MS2 a few years ago. This was a pre built, user installed system that was towed to my place to be initially programmed and then final tuned.
The biggest issue for me was getting all of the system info that pertained to this system. The problem is there is more than one software system to run the ecu. It seems that set up and tuning info for both(or more) control software systems is all mashed together in the same source. I was very confused on trying to set up the original configuration, because I didn't know there was more than one. I was looking for screens that didn't exist in the software I was using.
Once getting that straight... I really liked the system. extremely intuitive, simple to navigate, great data logging.
Now the holley. I have 2 complaints that I just need to get over. First is the "template" style format. I don't like looking for drop downs to get to icons. I realize the reason, but would rather have small icons I dont need on the screen, than to have to search for the ones I want. Big deal. I need to get over it.
Second, WAS the lb/hr tables for fuel instead of PW. This can be changed now, so really a non issue. Honestly though I got use to the lb/hr tables and again, I understand why it was done this way.
Holley's datalogging is very similar in operation to what megalog veiwer which I love. The custom icf tables are a dream come true.
I don't see any situation that I will recommend anything other than the Holley system for future builds/friends/customers.