To me this is all about scaling proportionately and correctly. For a long time POS's have been so overblown with HP, defenses and hardeners that it's not a content generator but the exact reverse. Small and medium POS have very little importance in cost or use, but they have so much HP that it makes it hard (if not impossible) for small gangs to do anything meaningful against the structures in the short term. Of course this is barring having your cap based ship orbit a POS for 24+ hours of non-stop shooting.
Anyways, the thing that will make or break this concept in my mind is pretty simple, how much HP will the structures have, as in how hard will it be to place them in a vulnerable state, how much of an offset will the structure going into a vulnerable state be, as in how long will it be re-enforced before becoming vulnerable, how will notifications of attack be sent out, and all the other small details. And secondly, how powerful will these defenses be in practice. Yes make the large and XL structures tough, but don't make the small and mediums overpowerful as well!
In short, give them too much HP (even for small to mid sized) or too many abilities or option for Defense, and they become non-viable targets for small gang roaming or engagements. Go the opposite route and they will never be used because they are too vulnerable/expensive no matter what the upside. I guess we will be looking closely as they roll out how the Entosis modules interact will everything, just as much these structures as everything else.
Like I said, sure making the Large and XL structures impossible for smaller gangs to attack effectively is GOOD, but doing the same with the smaller structures makes it that much harder for them to be content delivery and more to be PvP denial structures (much like current POS mechanics).
More good news though, at least ships will be off the scanner when idling inside!
See you in space.
OTHER USEFUL, RECENT DEVBLOGS
Sov Part 1
Sov Part 2 (Includes information on the Entosis module)
