There is absolutely nothing that an AEM tune gives you that a proper ROM tune doesn't.

Tuning an AEM yourself, while a bit better than just a safc and a wideband, still isn't as good as someone like Martin is going to be able to tune your car with a rom tune.

as far as the comment about reliability, again, that has to do with HOW it was tuned, more so than the equipment used.
IF, you are comparing a well done AEM tune to say, a mail order, "off the shelf" style rom tune, then I can see how the AEM could come out a head. But even then, it is a difficult argument to make considering a canned tune by someone with as much knowledge and experience as Martin, is designed purposely to be well within the margins of safety.
But, the flip side to that is, if you are tuning your AEM by reading the computer displays as you blast up and down the road, versus putting the car on the dyno and leaving the tuning up to a professional such as Martin, then the argument would be made that the professional tune is more safe and reliable.

Martin isn't your run of the mill tuner. People like me can only hope to know as much about tuning as someone like Martin has forgotten.

IF, you are highly knowledgeable when it comes to tuning your car with an AEM, AND you are the type that wants to "fiddle" with your tune yourself, then, I could justify spending the money on the AEM.
I personally would highly recommend the megasquirt platform if you were going the stand alone route, but I see the value in it.
However, if you are just wanting the end result, as in, you just want the car tuned, then getting the ROM tune is the way to go. Especially if you aren't planning on making any major modifications in the near future. There is the initial price of admission, getting the "chipped" ECU, but after that, you're just looking at an hour, maybe two, of Martin's time with your car on the dyno to tweak the tune.
On the dyno, you are in a controlled environment. Martin is going to be able to see warning signs of any issue LONG before you would notice anything on the street, so the engine would be more safe. Since Martin is so experienced, his initial set up isn't going to be too far off. It won't be like shooting in the dark, hoping you are within a safe tolerance.
When he does a tune, he is re-writing the entire fuel map, and many times, spark timing maps too.
He isn't using a piggy back to trick the Ecu to do something different or anything like that.

As for gains, it depends. He was able to take a 100% stock, Nissan Titan and without doing any modifications to it, tune it and get over 50 more HP, more torque, and better fuel economy.
the stock tunes are already on the rich side, and the stock timing is pretty conservative, he'll be able to optimize both to get the results you're looking for.

So, if you are talking about a ROM tune from some random tuner schmuck, versus an AEM, then yeah, I'd lean heavy to the AEM.
If you are talking about a ROM tune from Martin, versus pretty much anything else, it is really hard to justify the "anything else"

I would highly recommend talking with him and seeing what he suggests, and what you could expect out of a tune. I've had him tell me that it wouldn't be worth the tune for certain set ups since the gain wouldn't be that much.