A lot of sweet pea growers here-perhaps pop your question on there as well?
I use loo rolls successfully for lots of things. My OH wanted root trainers as a pressie last year and I got these "ROOTRAINER GROWING SYSTEM - SHERWOOD KIT - 32 x 12cm CELLS" and they worked very well for sweet peas tho' not sure what it is you are planning to grow.
Disintegration may be a problem but the 'mould' is usually not
I had same problem so thought I would try something else.
Have bought J. Arthur Bowers Grow tubes from local GC. Have no idea if they will work but a lot less pennies than root trainers.
I use loo rolls for deep rooting plants, not ideal, but if you pack them together tightly in a small tray they are less likely to disintegrate.
If and when I sow sweet peas early a deep pot with some cardboard sorts the root training. Cut Two piece of cardboard the width and depth of the pot then slit each half way slide them together and fit in the pot you have four mini pots but deep, Fill with compost and sow the seed two to a section and let them germinate take out the weakest seedling and grow the strongest on. I put the weak ones into a pot they often come out well.I often think we get too complicated and expect a modern aid for every purpose, my Father would say go get a wheelbarrow of common sense and use your ingenuity.