Very interesting to hear of this onwardflight.com site. I have traveled to over 70 countries and usually do not bother with onward tickets. I have only been asked by immigration to show an onward ticket 1x (Thailand) but have been asked by airlines numerous times (United, Copa, etc.). Luckily the few times I have been asked I had an onward ticket.
A trick I use is to buy a cheap bus ticket where possible. Oftentimes these are refundable and I buy the shortest one I can find (so let’s say I am flying into Bangkok, I would find a bus ticket to a neighboring country). The onward ticket you need to show doesn’t need to be air… just needs to show that you are leaving the country. this puts less money at risk and again, I’ve found I’m reimbursed about 85% of the inexpensive bus fare upon reimbursement. But this onwardflight site is another option although since it photoshops actual tickets… it’s more concerning to me than a real bus ticket.