I have one and never tried it, always had a trailer or platform (saw horses). I did modify mine for soft sand using an axle and tires from a Roleez beach cart.
Don't rest the hull on the dolly any longer than you have to as all the weight of the hull ends up on the four small padded tubes of the dolly. On the other hand, you can safely store a Sunfish on a Seitech dolly as the web strap that the hull rests on distributes the weight evenly.
Ok, here is my attempt at a dolly. I have only 1 sailboat trailer right now and I have 2 current use sailboats that share it. This leaves one on and the other on the lawn. I keep both covered with blue tarps, but the one using the lawn needs to be moved around every couple of days, or else the lawn gets ruined. So I built a dolly today.
I have been watching out for info and pics to help in my design. So I built a proto type (I'm always building proto types), I can change things as I go, or just sit on it for a while. The lumber I used was stuff kicking around the garage,( I keep an inventory of misc. lumber for these types of projects). I had 3/4" galv. threaded rod from another project from years ago, (as well as nuts and washers). I did have to buy the "wheel barrow tires" at Home Depot. So here it is, thought I was going to have to scribe a long keel holder, but it looks ok as is right now. I've moved it around the yard ( I have a sloping and bumpy yard), and to my surprise, the dolly stayed put! I thought that I would see a lot of slop and movement, but none. I barley had to scribe the short keel holders, and to my surprise, the weight distribution looks ok! I don't believe that there is too much pressure on the keel where it is supported. My long term storage (winter) does not include the using of the dolly.
Pretty easy, while the boat is on soft grass, I lift up on the side of the SF, and tilt it up. I then hold it up the hull with one hand and slip the dolly in with the other. I then ease it down so that it rests on the tire, and, slowly tilt it up on to both tires. I look carefully for any stress or strains, and have not seen any yet (I've tried it several times).