Canned tunes are always an approximation (best guess). There are variations between two motors just like there are variations between identical twins. Of course, this doesn't mean that the differences are going to be significant enough to cause problems. If I'm installing a $7k+ blower on my $50k+ Mustang then it's in my best interest to KNOW that the tune is correct. It's definitely worth another $100 to have it verified. The alternative would be to have my own AFR meter and the capability to data-log so that I can confirm it myself. Either way, it's cheap insurance to know that I've installed the blower correctly and that the tune is in the safe zone before I go pulling 100 mph burnouts through the local school zone. Hell, there are even differences in the fuel that you get in one place vs. the fuel you get from the same vendor just a couple of hundred miles away.
What doctor do you trust to tell you that your appendix needs to come out? The one that you went to see and could look at and verify all of the symptoms or one that you talked to over the phone that says he's done this a thousand times?
As far as my experience with canned tunes? Let's see... I installed my first blower some 20 years ago. Since then, I cannot say exactly how many others there have been or even what specific huffers they were. Like everyone else, I read the instructions carefully, research the things that I don't understand or are incomplete, and ask questions of people who have installed this unit to find out if there are any pit-falls.
Benefits of a custom tune come down to how good of a match the canned tune is for my combo. How close is the AFR to stoich at idle? Is tip-in adequate when I decide to mash the gas? How close is AFR across the entire rpm range at WOT? How close is it during moderate acceleration? How about cruise? Is initial and total timing correct for every load? If AFR is NOT correct, what adjustment needs to be made? Is it timing or fuel? All of things can be approximated given the same blower kit on two cars of the same make and model but how do you know?
If you do a search within this very forum, you will see some horror stories about folks trying to get motors replaced under warranty from Ford.
"SCT tune?", says Ford. Not a manufacturers defect and Not our problem.
"Sorry about your luck.", says SCT.
"Wish I'd verified the damned tune.", says car owner with blown motor.
These were just tunes. Not even a blower involved. The tune has be even closer than it does with an N/A setup. There's less margin for error when you start talking about forcing mass quantities of air and fuel into a motor.
I would hazard to guess that probably 8 out of 10 combos work very well with the canned tune. You don't hear as much feedback from them as you do from the other two. They're too busy making burnout videos for youtube, doing 100 mph through the local school zone.
It's not that the tunes are bad. The trouble is that since they are one-size-fits all, that they cannot reasonably accommodate the differences of each engine combo as hard as they try. Additionally, those tunes tend to be on the conservative side because the makers of these tunes understand that there are variations. They try and leave a certain amount of "slop" in the tune in an effort to prevent lean detonation. This alone, can cause some aggravation in relation to drivablity. It may not be engine threatening aggravation, but it sure is nice when the motor responds well to whatever condition it's being driven in.
There's a damned good chance that the tune received from Kenne Bell or SCT or (enter company here) is spot on. What it worth to you to ensure that? Can you predict detonation within the confines of your cylinders before it's too late?
Just because it's Smuckers, doesn't mean that it HAS to be good.