Thermal shrinkage alone makes it unsuitable for bigger double-sided boards.
There's a limit of practicality as to how big and how dense you can make your own boards, anyway. When the number of holes/vias gets too large, there's no point to make a board with the photoresist method, either. Well, I have seen the guys doing their own plated thru holes... and I dunno but it's not worth it to me, at that point, anyway.
You can easily make a double sided board as large as a laser printer can print, using a laminator, if you make your vias extra large. The warpage is a lot less using a laminator and pulsar paper vs using an iron and say magazine paper. When the number of vias goes up, I will prefer extra large vias, anyway, to use premade tinned via tacks. Getting the registration right takes a little more time than with photoresist, but the registration is a bigger concern than warpage, when you are using quality tools and consumables and reasonable pcb sizes for toner transfer process.
Individual needs vary. Speaking for myself, I find no reason to use photoresist over toner transfer for any practical purpose. Toner transfer is faster and easier and cheaper and is > 99% yield at, say, 10/10 and up to 6"x9" when you use the right tools and process. I could easily do panels of the board in the OP up to 6"x9" with 99+% yield, anyway.