Sorry for being so slow in replying – missed it first time around.
The drawbacks of the 45 degree leg press are that you are forced to curl as you get into and out of the seat (unnecessary forward bending of the spine). This is not an issue with a more horizontal seat. Also, some of the leg press units do not have a head support and for many this can place a lot of strain on the neck.

Hello,
Great question. The merits are so minor I cannot think of any!
The drawbacks of having the seat at a 45 degree angle are:
It is difficult to get on and off the seat without “slouching” or coming into some forward bending of your upper back.
Most seats do not have head support and therefore more stress is placed on the neck musculature – remember to keep your tongue to the roof of your mouth!
Either way, the leg press, works your lower body but does not have the added benefit of getting your back muscles involved and stronger. It is also less “functional” in that it does not teach your body how to prepare for heavy lifts that you might encounter in your day to day activities i.e. lifting kitty litter bags etc.
Be well,
Margaret