No, and Yes.
No, most pinball machines are not "investments". No business journal I've ever read or financial adviser I've ever talked to ever recommended pinball machines or any "collector" hobby as an investment. Too much emotion, volatility, and- like someone already said- irrational behavior involved. I say "most", not "all", since I do see the super rich (at least I hope they are- for their sake!) buying LE's and/or super rare Pins that have huge upside potentials, but also huge downside risk. These people, I assume, already have healthy stocks and bond portfolios and fully funded 401k's, so... why not?
Now, since I think the "drift" of your question is... "can I call a Shadow an "investment" and reasonably assure my wife that it won't lose us alot of money?"... Yes, I say absolutely. The Shadow in particular is one of those pins where supply and demand has settled out for some time now, and basically they get passed around at roughly a constant value. Decide you want one?.. you can land it at the price you want within 30 to 60 days. And even better, because the price of new pins are steadily (and sometimes shockingly) going up, then yah! for used "good value" pins like the Shadow.