re: European Mission Work, Worthy Cause or a Scam?

In this particular instance, not a worthy cause, but there certainly are other organizations doing great work over there that aren't scams. Just like spending money on a product or service, it's important to do your homework before giving money.

I'd also add that typically, donations given directly to an upstanding organization are more impactful than donations that enable a friend to volunteer their time for that organization... Unless of course the friend has a specialized skill set that's urgently needed. That said, I've done volunteer work with refugees abroad and worked alongside some people who were funded by friends and family. It was their only way to afford it and they didn't treat it like a vacation. I'd consider the donations well spent... just could be more effective dollar for dollar if given to the organization directly.

ETA: my post refers to actual volunteer work, not missionaries focused on spreading faith.