Dusty, I bought a new house and moved into it in July, 1977. At that time, instead of hydro seeding, they called it hydromulching. I was trying to decide between the slow seeding method I'd used before and the quicker, but more expenisve sod when a friend who was in the business of building new golf courses told me about hydromulching. So that's what I did and at that time it was about half the cost of sod (if you don't count your labor for anything in putting down the sod

).
The first thing that happened was what you had; a big rain washed seed, dirt, and all onto the sidewalk in the front and the paved alley in the back.

I got my shovel and rake, put it back and raked it fairly smooth, but of course now the seed and green mulch don't show on top. So I called the company that had done the job, and had guaranteed it against such problems. The guy said they would
prefer waiting two weeks to see which areas were growing and which needed additional treatment,
but if I really wanted them to, they'd be out the next day. I agreed to wait, and sure enough it came up everywhere so thick that I never called them back.
Remember, this was July in Texas; it's hot. And we used common bermuda. The instructions were to water it 2 or 3 times a day, preferable keep it damp, but not enough to run off. At the time my wife was not working and school was out for the summer, so she and our daughters didn't use a sprinkler. They hand watered and kept the yard damp. The instructions said to mow and fertilize the 30th day. Hmm, it grew so fast and got so tall that I couldn't wait that long. I think it was about the 21st day when I mowed it. At least three neighbors who moved into that development after we did, came to ask how we got such a beautiful lawn so quick. Then they called the same company to do theirs.

As far as I'm concerned, it's definitely the "best" way to go, but bermuda needs sun and heat, in addition to water. So I don't know about other grasses. One neighbor moved in in September and when he called, the company told it was too late for common bermuda, but they could do it in fescue, and did. I don't know which fescue they used, but that stuff spread into my yard, grew twice as fast as bermuda so it needed mowing more frequently, made a pretty lawn as long as it got plenty of water, but was just clumps when it didn't get enough; horrible stuff.