Is it safe to fly on airplane with lithium batteries in cargo hold?
No, it is not safe to fly on an aircraft that has lithium-ion batteries in the hold.  There is a commentary on FAA Safety Alert for Operators (SAFO) 10017 here, which was issued after the crash of UPS Flight 6. 

Admittedly, this was a bulk load of 81,000 batteries on a cargo flight.  However a search for 'battery' on AV Herald shows between one and two incidents each month, of lithium ion batteries/devices igniting in the cabin.  These are only the ones reported on that website, we don't know how many may go unreported

The ICAO also considers Li-ion batteries a risk but as Simon Hradecky points out, there is a conflict between this safety advice and the security requirements that force passengers to check-in laptops, tablets etc on some flights.

Personally, I think that, whatever the ICAO, FAA and airline requirements, there are going to be some of these devices that get into the hold (people will forget that they left a battery bank in a bag and security will not recognise it on the scanner). We know that they regularly catch fire.  There are almost certainly going to be airliners brought down by this, it may have already happened - the EgyptAir MS804 fire hasn't been explained yet.