David Austin in his early book titled Old Roses has a lot of good things to say about them, pointing out how " they thrive on neglect." What I took away from that was that in order for modern roses to bloom generously and repeatedly, they need a lot of feeding. This it doubly true if one is pruning a rose severely in order to promote bloom. I like the idea of roses being able to fend for themselves, even if I obsess over them as they settle in.
It seems to me that there are roses that bloom once but manage to produce more blossoms than the typical hybrid tea rose does in its lifetime in a garden. Ballerina comes to mind (I've grown it in two zip codes and have never seen it repeat) ditto Sea Foam. When I grew Great Maiden's Blush in NJ it covered itself in flowers for a glorious two or three weeks. I've seen it blooming beside a full-grown Cardinal Richelieu that was also in full bloom and the pair took my breath away. Not a pair of roses for a small garden. And then there is Ferdy. It's a modern rose, but a once-bloomer that does literally cover itself in bloom. It's the once-flowering climbers and ramblers can be even more glorious. Sometimes I think it is better for a plant to put on a big show, then disappear gracefully into the background than to keep on looking just a bit silly as it doles out a single flower at a time. And I'm afraid I care a lot about how plant foliage works in the garden. I've never chosen a rose solely on that basis; but it does affect my decisions about placement and shovel pruning. I love viburnums for their foliage, too.
I've been pleasantly surprised by a few repeat-flowering roses. Rainbow Sorbet has produced flowers continuously since May, and it has been pretty generous with the display. So have Coral Crown and Europeana. Crocus Rose has had two conspicuous flushes of flowers and has not been without a bloom since May. Things go downhill from there. For example, Legends - in it's first year has produced one rose every other week. Duftzauber 84 which is in its second year and is more than three feet across has had two flushes of bloom, each of which produced about eight perfect roses. But the roses opened quickly and fell from the plant in about two days. So I've gotten thirty two bloom-days out of it. It's the most generous HT bloomer I have. Ispahan, though, probably produced eighty or a hundred and twenty blossoms in May. The flowers lingered for weeks. If we said the average flower lasted eight days, then Ispahan produced about (8x100) eight hundred bloom-days: almost thirty times the garden impact of my best HT rose! Why do I not fill my garden with Ispahan roses? I like to have roses through the summer and early fall. But I do think that sometimes you actually get more bang for the buck (flower impact for the space used) with old garden roses or other once-blooming plants. As much as I love hybrid tea rose blossoms individually, the more I learn about roses, the more reservations I have about using them as garden plants.
As for disease resistance, one of the many reasons I left the Northeast was that blackspot definitely does kill roses there. It kills most hybrid teas. It killed most of the minis I planted. It killed some floribundas. Most old roses and modern shrubs did fine. There was not much problem with powdery mildew, gratefully. I understand that bourbon roses tend to be among the most disease-prone roses. Except for bourbons, I'd expect most other old roses to be far superior to most of the modern roses - especially the ones bred on the west coast during mid-century. Albas, Gallicas, and Damasks that I have grown have been almost completely bulletproof. It may very well be that modern roses - especially the glossy-leafed ones bred on the west coast - are more resistant to mildew. My guess is that the Kordes roses would tend to be better than the Meilland ones as disease resistance goes. But, then, I was surprised to see Folklore's leaves being deformed by the early onset of powdery mildew recently. I guess you can use generalizations to help simplify the search process, but I suppose the actual attributes of the rose cultivar matter more.
