I've been recommended against doing this (using a free subdomain) because if the site ranks high on search engines and you decide to switch to your own domain you have to do all the SEO work once again to get the good domain ranked.I use free hosting with free subdomain (preferably 000webost) for new websites with a little magic SEO.
If I see success coming, I transfer everything to my paid hosting and buy a new domain (with the free subdomain redirected to the new domain).
And real hard work SEO, tactics, promos, giveaways, hot content and other strategies to make it more successful.