Same holds true for substitutions in crocheting and knitting. There's a techie saying "garbage in garbage out" and that applies here as well. Why would anyone want to take the time to make something to have it turn out lacklustre, ill fitting, the wrong size, etc.? Not me...that is unless I'm experimenting and letting the craft take me where it will. However, if I'm putting in the hours and concentration for a hand crafted item that I want to turn out "perfect" or at least "correctly," I am aiming at its best result and "doing my homework" before I make substitutions that may not work.
Of course, you can take a fine thread doily pattern worked with a small hook and substitute sport weight cotton and a larger hook, knowing that the result is going to be LARGER. Instead of a doily you have, perhaps, a table topper. Good. Nice. Fine. You didn't however, use those larger supplies thinking you were going to get the same result as the pattern intended, right? Right! A little research is necessary before substituting...by intention or necessity.
I had a student recently come to me with a rug pattern. (You know who you are....lol.) It was vintage. It was ancient. The company had gone out of business years ago, the yarn had not been manufactured in...forever...and PROPER substitutions had to be made for the item to work out .... correctly. My personal first inclination with rugs is to work them in cotton yarn. Of course, wool or acrylic can be used...that is YOUR decision, and is suggested in the pattern you choose to follow. Caron currently makes an acrylic "rug" yarn that it believes rivals acrylic rug yarns from by-gone years ago. I don't have personal experience with Caron's rug yarn (disclaimer) and am not pro or con as to it. The point is, that step 1 is to make that decision: what kind of fiber is appropriate and what is YOUR choice? Step 2 is choosing a similar yarn weight. Step 3 is....getting enough yarn to complete the item. Do your research. How many yards did the original pattern require? Step 4....use the correct hook or needle size. Step 5: Does gauge matter? If it is meant to FIT, it matters and you'll want to work a gauge swatch before beginning your piece.
Pulling all of the above information together will assist in finding the correct substitution for your item. With the proper supplies, luck isn't required...but I wish you luck anyway. :)
Happy crafting, Donna