I think I'll just give a few of my thoughts.
I've seen short clips more recently...but last full game was January.
I think he would benefit greatly from playing in a solid junior club with good facilities where he can work on individual skills and technique for another year or two. When we talk about 17-18 year olds who play senior level hockey and thrive we're definitely talking about the minority. Players that are both fundamentally and physically developed. A player like Matthews, Laine, Puljujarvi, or a Nando Eggenberger etc.
In a professional hockey club they expect you to have a certain level of proficiency in various aspects, be it your power-skating, stickhandling, passing, shooting, or physical play/pursuit. Playing pro-hockey in your teen years can definitely be helpful for developing your physical play and decision making. But in order to benefit from such one needs to have that expected level of proficiency in the various technical aspects of your game and probably be above that minimum requirement in at least one or two categories. Pro-clubs are focused more on winning and less on developing athletes and Bregenz doesn't have a lot of the resources a player might want to tap into when developing specific fundamentals.
Fassler definitely has things going for him where some of the intangibles are concerned, but physically he could use some work and fundamentally he could also use some work as well. I think the next season or two and off-seasons will definitely be important and indicative for his long-term development.