Why should I trust this service?
You shouldn't. The simplest check is to download the original source your coin was templated from and compare the two. Carefully consider any differences you find.
This is the risk. Downloading from an unknown or not trusted site like this one. If may or may not be scam, but its better for as to be careful than sorry.
I think a far more common risk would be a backdoor in the source and thus the coin. Not necessarily my computer.
It's a website where you only need to select the settings, and it will generate an altcoin + wallet for you. Seems too good to be true.