Fuel Cell vs Fuel Tank

And there are a variety of 'fuel cells' from plastic to a metal can with a bladder.

The plastic ones are for use on track and are properly installed only if FULLY separated from the passenger compartment occupants by a full metal bulkhead, as in a sealed metal barrier behind the back seat or mounted under the car. (I personally would never put one of those in a trunk...)

The full metal can with bladder is the safest but they run around $500-600. Those are the only one to even consider putting in the trunk IMO, but even then, that is not the safest thing to do for a driver car.

Filling a cell in a trunk presents issues: Filing in the trunk invites fuel spillage and the smell and odors. Using a fender mounted filler has its own problems, usually at the joints which are often done with rubber line sections; these are prone to tearing loose and spilling fuel in an accident.

Cell mounting and anchoring in the trunk in another area of concern. A full 15 gallons will easily weigh over a ton at peak deceleration in a hard frontal crash (with >20 g's peak deceleration) , so the use of steel straps or tubes over and around the cell is necessary, along with anchors to the floor to support that. You don't want one coming forward to join you in the passenger compartment...

For safety, a standard tank under the trunk with an all metal filler tube is hard to beat for a daily driver. The spilled fuel in a wreck is at least generally outside the main car's bodywork. Fuel cells have a good place in racing but they can be poorly installed and have their own safety considerations. Please accept my apologies if I come across too strongly on this... I have been a racing tech inspector for 15 years and have been taught a fair amount about how to do this well .... and have done it poorly myself years ago and learned to correct it.