Of course, big old plants of 'Fortuniana' are common, probably nearly always the remains of a formerly grafted plant. I wouldn't hesitate to recommend any of these roses on their own roots, for this area of the State. But I would not recommend any other own-root roses for Central Florida, unless you know that the variety has been grown successfully, for many years, or unless you are willing to replace the plant fairly soon. If you want to gamble, other Chinas seem most likely to be successful, followed by some of the Teas. Somewhat north of here, where the clay content of the soil is greater, the situation changes.
Many roses do well there, on their own roots.
This is probably because root knot nematodes do not thrive on heavy, clay soils. I think it is a very important observation that you just don't see old own-root bushes of other varieties in this part of the State. There must be a reason for that, and I think it is that the plants have all died, due to their inability to adapt to our soil and pest conditions. Many people are planting Tea roses on their own roots, in this area, now. I wish them luck and success, but until those plants become several years old and are seen to be still thriving, I won't recommend own-root Teas for this area.