I am familiar with the general charge, of course. My own experience is that my heirloom tomatoes have done as well as any of the hybrids I've planted. I seem to remember that Carolyn says in her book that the danger is exaggerated.
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I'm not sure I used the word danger although I've certainly said that most OP's do as well as hybrids when it comes to disease and that was mentioned above by someone, the reason being that THE most common diseases of tomatoes are the foliage diseases and hybrids aside, with few exceptions there are no genes that have been IDed that impart strong tolerance to the foliage diseases.
Suze, Feldon, TZ and others have said about the same as I would have written.
Just a couple of points.
If OP's, heirloom or otherwise, were so susceptible to diseases we wouldn't have all the Livingston varieties, the Henderson varieties, even Trophy from what, 1860, Green Gage and Roi Umberto and the small red and yellow pears, all pre-1800 varieties, with us today. And I'm leaving out lots of other seed companies that were responsible for introducing some wonderfuol varieties from the late 1800's to maybe 1930 or so.
The genes for many tolerances incorporated into the newer hybrids came not from our garden tomato, rather, from some of the other species and Keith Mueller has noted those at his website.
As mentioned above, and I'll kind of repeat it, hybrids were never developed specifically for the hobby gardener, they were developed for the commercial farmers, except for more recent varieties such as Brandy Boy and Glory and I think there are a couple more for the home gardener.
Carol, I can't remember which article you're talking about unless it was the one I wrote that was published in National Gardening when it was still alive and well. It's still available and I have it in my faves somewhere b'c when they went under due to the parent Co going bankrupt they chose certain articles and asked permission to put them as links at their new sites and I think paid each of us $25 for that permission. I spent that ASAP.
Some of the greatest satisfaction I've had when it comes to tomato diseases has been working with the Cornell Coop extension in the two counties where I've lived and gardened. Most specifically when I was growing at the old family farm and the head of the five county area around Albany, NY, wanted to do a disease survey comparing diseases between OP's and hybrids, so I breathed deeply and planted some hybrids. They had student interns that were supervised doing the Iding, with supervision, and recording the data. The head of that Coop Ext, Dale Riggs, had learned about Iding diseases from Dr. Tom Zitter at Cornell who is a tomato disease infection specialist and I learned more from that summer's growouts than I ever knew before in terms of IDing diseases, most of whch were foliage diseases b'c aside from a bit of random Verticillium there aren't that many soilborne diseases where I live and garden.
OK, typed too much, but I agree that there are still lots of folks out there who have no idea what they're talking about, didn't Feldon say idiots? So that's why it was so satifying to me to speak to garden club members and at Master Gardener classes and hold field demo days as well as presenting info to farmers wanting to get into the heirloom tomato business, this via the Cornell Coop Ext, so that some of the ignorance could be lesseened by actually seeing compariosns of OP's and hybrids.