Linda, the owner of Long Ago Roses, runs an excellent operation. The quality of her plants are generally very good. I wouldn't hesitate to purchase roses from her, if I were in the market.
As for 'Rainbow', it does grow better when it is a budded rose, but it can grow into a solid plant as an own root rose. It takes a long time for this rose to "get it's feet under it" and then it's a solid rose.
Most people hate to do what this rose needs in its first years in the garden to help it along because the blooms are so beautiful. If you don't allow the rose to bloom for at least two years, it will put its plant energy into growing roots and plant. That investment pays off big time.
It takes a lot of plant energy to create and maintain blooms. It's normal for this rose to have spindly growth while it is a juvenile rose, but once it's happy, the canes get thicker and stronger and it's a beautiful rose.