A few observations:
1. In the past the advice was to face the SPG away from you when pressurizing the regulator, in the event the face blew out of the SPG. That's ancient advice as modern SPGs won't fail that way.
2. In the even more distant past before the earth cooled, the high pressure port and HP hose did not always have a very small restriction in them so it was possible to slam the SPG with high pressure gas. Now however, both the HP port and the HP hose have very tiny orifices that won't allow enough gas through to either slam the SPG with sudden high pressure, or cause significant gas loss in the event of an HP hose failure.
3. As noted in the above post, the reason for leaving the purge button slightly depressed when turning on the tank valve is to prevent the knife edge seat on a piston reg from slamming into the seat, accelerating wear. That's semi-old advice as seat materials are much better now and true knife edge seating surfaces are much less common.
4. On the other hand, the practice came back into fashion with high percentage nitrox mixes and O2 mixes. There have been several instances of first stage fires and even a couple that catastrophically failed when O2 was used and the valve was turned on quickly without the purge being depressed.
In this case, the problem is adiabatic heating of the oxygen as it enters the reg and then is suddenly compressed. High flow rates around sharp edges as the gas floods into the regulator can also result in hot spots. In both cases, if you get enough heat to ignite the O-rings, you'll have a first stage fire. EPDM and Viton are used as they have higher ignition temps than nitrile O-rings, but they can both still be burnt if the temperature is high enough. Viton has a higher ignition temp than EPDM, but when it burns it produces phosgene gas, which was used in gas attacks in WWI and isn't something you really want to breathe.
Most are brief and usually not even noticed other than the first stage suddenly leaking. I've had few first stages over the years come in with charred O-rings and that's essentially what happened. However There have been some more spectacular failures.
So for air and recreational nitrox mixes, I wouldn't worry about it, but for O2, it's prudent to turn the tank valve on slowly.
5. As for reliability of SPGs versus air integrated gauges there are some significant downsides to air integrated gauges and electronic SPGs:
a) they require batteries where an SPG does not. Beyond the obvious issue of limited battery life, there are other potential issues. A long time ago, electronic SPGs made an appearance and I tried one for awhile. At the time I was doing deep, cold water dives and I found that with bottom temps of 35 degrees F it was not uncommon for the SPG to quit when it cold soaked and the battery voltage dropped. It's not pleasant to look at your SPG and see a blank display because it got cold and decided to pack it in.
Some air integrated computers have similar issues, and again way back in the day in very cold water I've seen them fail at depth despite not having any low battery indicator before the dive.
b) You're also dealing with greater complexity and set yourself up for a single failed electrical connection failing both the SPG and the computer. The shop I worked with also broke a boat load of Sherwood Wisdom computers starting from a 100 degree boat deck and going to 35 degree bottom temps in a matter of a couple minutes. Apparently the expansion and contraction involved inside the computer broke connections in them. Sherwood actually called and asked what we were doing to them since we'd had so many failures. I hope they improved the later iterations, but who knows?
c) Mechanical SPGs may be a couple hundred psi off in one end of the range or the other. However if they are in error, they almost always read low on the low end of the scale, as an SPG that reads high at the low end of the scale will have a needle that won't go all the way to the zero stop, and thus people notice the problem as it's reading something higher than zero when there is no pressure on it.
Electronic SPGs will fail the same way. I've seen electronic SPGs and air integrated computers read between 200 and 300 psi when not connected to anything. If you're lucky it'll still be under warranty and/or will be supported by the factory. I've also seen the transducers fail low and reach zero psi well before the regulator stops delivering gas.
The difference then is that mechanical SPGs tend to be what they are in terms of accuracy over the scale from 0-4000 psi and are very constant. For example, I have one SPG that will read 3800 psi while it's twin will read 3600 on tanks filled on the same manifold to the same pressure. They are both accurate and consistent with each other at 2400-2600 psi where I normally turn a dive, so it's not an issue, and that SPG has been that way for years - it's very stable.
Electronic transducers on the other hand seem to just decide to go bad and you won't always catch it when it occurs.
6. "Accuracy" differences. It's harder to extrapolate pressures between the 100 psi hash marks on most SPGs, while air integrated computers and electronic SPGs will often display pressure in 10 or 20 psi increments. The thing is that the more granular display isn't the same as "accuracy". If it's 100 psi off, it's still wrong even in the number is more precise.
On the other hand the trend information is more useable, and it has to be if the computer is also figuring gas consumption.
However, if you have a regulator issue (obstructed filter, partially opened valve, etc) you'll notice it with the needle swing on the mechanical SPG, where an electronic SPG will either average it out and display a constant value, or it will sample at fixed intervals where the pressure may or may not be accurate depending on the pressure at the HP port at the time, potentially producing fluctuating pressures that you'll have to compare against each other.
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In short, with all thing considered, I see zero value in an air integrated computer or an electronic SPG. Mechanical SPGs are simple, reliable, affordable, are quick and easy to replace if they break, don't require batteries, don't mind cold water,and will quickly clue you into problems such as a partially open valve where an electronic SPG won't. A mechanical SPG will also continue to function if it floods, where an air integrated computer or electronic SPG won't.
