This is just a random observation, but I had an interesting experience recently. Since early this spring, I've been mixing my own home-grown seed starting soil with sifted homemade compost (sometimes still barely thawed), sand from the kids sandpit (playsand), and good top soil from under the leaf pile/composted wood pile.
On occasions, I didn't have the sand or top soil on hand so I just used 100% finely sifted homemade compost. On one occasion, I didn't have anything on hand (too lazy or too rainy to go out and get more), so used pure bagged compost I had in the garage -- one called Bumper Crop and one mushroom compost from local source.
Now, MY compost is cool/warm pile so it's full of weed seeds that I just pull out as they sprout (DEFINITELY need to know what the sprouts for the seeds your planted looks like or be able to recognize the weed sprouts ) It's also full of critters -- worms, nematodes, thread worms, centipedes, pillbugs, who-knows-what-else.
I was pretty sure that the Bumper Crop, at least, will give me better results than my own compost, but to my surprise, the seedlings -- cuke, watermelon, honeydew, morning glory, etc. began to yellow soon after true leaves began to grow. Had no problem like that whatsoever with my own 100% compost. I could understand it with the mushroom compost -- it's SPENT compost after growing the mushrooms afterall, but the other one (especially with a name like that)? Hmm....