Strainer stock works fine for me and has for hundreds of prints, large and small. And in this style of market, strainer stock is by far the most common form of framing material used. There are good and bad brands and some can give over time, which will allow sag. I use the Larson brand, part number is 6011. Strainer works much better with larger prints, say 36 x 72 or 36 x 96, 24 x 76 or 30 x 76. If the frame is made correctly. I am very familiar with the the keyed style of frames, I much prefer the strainer stock style of frame.
With strainer stock, you need to make sure you have added both a center bracer bar, 20 x 30 and on up, and on anything larger than 24 x 36 I add corner bracing.
Other issues may be the canvas itself? or stretching methods used?
I print on Breathing Color canvas, either Lyve or Crystalline and use the Stretch relief pliers for my work. Many feel that they are too expensive, however they give you excellent control on the canvas and a very tight stretch. I don't feel you can get as tight a stretch with standard pliers like the fletcher style. The results with the stretch relief will rival a machine stretch.
Just one more opinion to consider.
Paul