No. Everything is in degrees. A Ruger LCP is not as easy to get good hits with as a full-sized pistol, but it's still operated the same way with the same aiming form.
It's marginally less practically effective than a full-sized pistol however its compact nature is quite literally a crucial trade-off for those folks who can't carry a more "shootable" gun in some situations. It's size is a benefit so worthwhile that one might have NO gun if one didn't accept this trade-off, and the hit one takes to shooting effectiveness is small enough to be a good exchange.
A PGO shotgun is not marginally less effective in the hands of a defending shooter. It is seriously less effective. This has been tested extensively. A conscious decision to choose to defend yourself with one, then, needs to have some VERY important trade-off value that's more important than speed and accuracy.
Some have said the ease of storing one for transport is such a worthy trade-off. Ok, but I'd simply choose an effective handgun instead because I can put a lot of hits on target with one far more surely that I can hit with a PGO. That's an evaluation you must make for yourself. But do so with DATA, not assumptions about how you can perform.
Well, that truly depends on what you're doing with it. Either one might be "useless" for some tasks.
That depends on whether you've got a "brace" style stock on it so you can shoulder it or not. Having run many (unstocked) rifle-pistols against timers I can say they really aren't something one should choose to take to a gun fight. They're slow to get on target, unwieldy, and abandon the natural aiming traits of either a handgun or a rifle. A standard handgun is an almost infinitely better choice in any normal "self defense" setting. A 30 round mag of 5.56 does not trump 9 rounds of .45ACP, when the pistol can be drawn and put aimed hits on target in a second or so, and aimed follow up shots are coming at a rate of 5 per second. An AR-15 shot as a handgun just doesn't work like that. Can make a fun groundhog plinker though.
If you haven't run trials between these different kinds of guns these concepts are hard to grasp in scope. If you do, the differences become pretty stark.