Yup, no manual add valves would make it a no go for me too.
I think the thinking is to force you to bail out onto OC, hence the BOV on the Sentinal, and the loss of manual add valves on their shallow water version.
Makes the units purely aimed at people who are not willing/interested/able to learn to manually control their units.
Perhaps by making a very simple bailout 'the solution' to all possible problems, makes it a more acceptable package to sell to recreational divers and agencies and hence break into a large (and lets face it money making) new market for the manufactures.
At least on the Sentinal the middle and top versions have the manual add valves
.
As for manual add valves failing more than solenoids, well I think Deecoweenie hits the nail on the head by pointing out that maybe solenoids are very reliable but controller boards and power failures happen far more frequently.
As an aside I have a diluent solenoid on my MK15.5 now and a switch on my primary to allow me to 'electronically but manually' add either O2 or Dil by sliding the button one way or another. Very nice, convenient and super quick, especially when busy working or have hands full of lights, cameras etc, but its never, ever going to replace the manual additions on the unit. They seem a basic necessity for a safe and capable unit IMHO.