The biggest disadvantage of the cross arm style is it is difficult to quickly counter punch. Foreman used a cross arm style up to and thru the Holyfield fight. After the Holyfield fight George returned to a more traditional style due to specifically this issue.
The only thing i wanted to add was that Archie Moore found a way to make a right cross work for him in this stance. In the Rocky Marciano fight he demonstrated a sharp fast right hand lead counter that he delivered by leaning over to his right, building power and then springing forward using the potential energy of his weight being on one side. It's obviously effective as Moore was 1 of only 2 people to drop Rocky. I would say the best way to set it up is by not using the right too often during the round. Think of it as a concealed sawed off shotgun which has limited ammunition. You don't swing it around or brandish it revealing your secret weapon. Lure the opponent into a false sense of security spending a lot of time blocking while leaning to the right and waiting for the perfect time to launch the right on an opponent who is coming forward aggressively. You can also set up a powerful overhand right from the crab stance. I would suggest watching Ken Norton and Tim Witherspoon they made it look easy. The Philly shell is more of a slickster's tool. The stance is better suited for using jabs and pull counter right hands while it is harder to use jabs and straight rights in a crab stance. The way Floyd Mayweather and James Toney use it is with the body held standing almost to the side with the lead shoulder facing the opponent. This makes it very difficult to land blows to the head since a Philly shell offers an extra cushion to minimize the damage from a blow. It is recommended to tuck your chin and be very alert. Ideally you don't want to just stand there letting your shoulder absorb damage on purpose, if you see the punch coming it's better to get out of the way or use the "shoulder roll" and sort of parry incoming shots with upper body movement and head slipping. Mayweather is the undisputed king of deflecting and avoiding shots with the Philly shell. The difference from the crab defense is that it requires a bit more speed and athleticism as you are holding your feet in a more awkward side stance. You can be wide open for big sweeping/looping shots as your hands aren't held as tightly together as the more fortress like square on crab defense. Marcos Maidana showed the flaw by pushing an aging Mayweather to his limits with his relentless slugging and volume punching. The Philly shell user wants the fight to be more like a chess match and at a pace that he's comfortable with. This is because standing at a horizontal angle requires good footwork and a "bird's eye view" to be aware of you and your opponent's position in the ring so you don't get trapped or caught by a big shot. In contrast, a good crab defense would have no trouble fending off looping power shots as they often come one at a time and the slugger would quickly tire. It would be like hitting a heavy bag. The crab user doesn't move his hands around as much and is more power oriented while the Philly shell user likes to use feints and shifty movement to time opponents with counters and slip out the way like a matador with a bull. The Philly shell user parries and deflects left jabs with their right hand or catches them. The opponents right hand punches are deflected with the left shoulder. In theory it should be almost impossible for a conventional straightforward 1-2 Orthodox fighter to land a clean blow on an expert philly shell user, especially if he lacks speed and ring IQ. The user's right is also held high near the chin to discourage left hooks. The left arm is held across the body for protection. Unlike the crab defense, the Philly shell combined with good upper body movement and hand speed can actually allow for sneaky lead check hooks or hooking off the jab. With the left held low it also makes it easy to throw a spear like jab to the opponents body. Mayweather is an expert with all of these. One weakness of the Philly shell is to southpaws. This may seem strange as Mayweather defeated several south paws but he is the king of this style. The way the hands are held would usually leave you open for a straight left hand or a looping left hook from a South paw stance. The Philly shell also leaves you with a bit of a blind spot on your left as you can become too comfortable and confident that your shoulder will be able to absorb and deflect attacks from that side. A skilled body puncher might be able to dig in with a right to kidneys or a right hook to the head. On offense Philly shell users can be surprisingly good inside fighters. Like the cran defense they are already in a good position for body shots. Unlike the crab defense, standing off to the side awkwardly cab make the Philly shell user's intentions even more ambiguous as the opponent literally can't see their hands most of the time at close range. James Toney was quite the beast with his nasty uppercuts, right hand counters, and ability to go to war with short sharp blows all while making the opponent miss by mere centimeters. He lived by the expression "he could fight inside a phone booth"--able to find room where most boxers would complain to generate offense. Also, the shoulder pressed against a squared up orthodox fighter's chest made for an extra level of protection and disguise as the Philly shell user plotted to land something sneaky. The fact Floyd Mayweather and James Toney were never KO'd despite fighting all kinds of boxers across multiple weight classes highlights the effectiveness of this stance.
Right hand to the body, left hook to the head is key for breaking down a Philly shell guy. Your left hook keeps that counter right hand at home getting behind their lead arm naturally starts forcing them to square the stance a bit. At this point a fighter will either abandon the shell, or get stuck inside it, unless you are James Toney who was just that kind of comfortable.
IMO, this defense suited him as he was too slow to counter punch in his second career. The Philly shell and crab style are all but extinct at heavyweight these days. Given that most heavies don't have in-fighting skills these dyas it could make for a fine change of pace tactic for the shorter guys to use.
It's designed more to absorb impacts rather than redirect them, so yeah it was perfect for old George.
