For me it depends on the gravel or sand. The sand in my axlotl tank is play sand and has a fairly large grain size and was fairly easy to plant. The sand I recently put into my daughters new aquarium is another brand of play sand and is WAY to fine! It's like trying to plant into dust, and plants just pull straight out of it with no resistance at all.
Gravel is good if it is of a nice small size of about 1-3mm, over 3mm and it is too large for me making it hard to plant. It's also tough on your fingers when planting (not as big a problem using tweezers, but some plants are too big to plant with tweezers. lol).
These days if I was going for a 'plain' substrate I would go for one of the rounded baked clay type ones to be honest, they are much kinder on your fingers and give plants something nice to anchor into.
I've just started planting into some eco-complete in a shrimp tank I am setting up though, and I have to say it's the nicest stuff I have ever planted into (so far) as the grains are small but not powdery, and it has a dark black colour that I really like a lot. If you get eco-complete though ignore the 1 bag to 10 gallons that some shops say, I reckon at least 1 1/2 bags per 10 gallon is more like it, as 1 bag doesn't give the depth that I prefer. I know that ADA AS is superior, but I don't like the look of it much, hence I went with EC.
Ade