My take is that 10+1 pulls should guarantee a 5* minimum, but only if NG is used:

1) NG is the only way the devs get revenue, which keeps the game alive. Remember Aigis? Guaranteeing 1 5*/6* unit provides decent incentive to spend without making it lopsided.

2) FGs aren't exactly hard to come by; players are guaranteed at least 40/month if they 3* all event/reissue maps and log in at least 24 days each month. That's enough to provide 9 10+1 pulls in a year (almost $400 worth of gems). Not nearly as generous as DMM by all accounts, but that's still a decent amount for free. Currently, a 10+1 pull has a 50% chance to have a 5* or better unit; it definitely sucks when you're on the wrong side of that 50%, but it's still a decent shot considering it's free.

I totally understand that many people would like to see a guarantee extended to the FG pulls as well, but I'd also like to see FKG stick around longer without risking eventual closure.