Rather than jump quote you here, let's lay some things out. Flash not only has security issues, but it's a resource hog forcing computers to work harder. Performance issues are tantamount. Flash also has major memory corruption/leak issues. Everything bolded is a fact. It's is a platform that's able to be used on MAC, Linux and Windows, so a cross-platform, complicating it even more.
In fact, 32% of all corrected bugs in Flash have to do with memory corruption. A look at every corrected issue and vulnerability over the years is compiled here: http://www.cvedetails.com/product/6761/Adobe-Flash-Player.html?vendor_id=53
The problems with Flash go deeper than just "security" issues, as you stated. Here's an overview of the vulnerabilities that Flash has had in the past 10 years:
What's the issue?
"It's usually the legacy code (going all the way back to AS1 targeting much, much earlier player versions) that gives most of the problems."
This article explains much of the problem. https://www.quora.com/Whats-wrong-with-Adobe-Flash
I'll quote some relevant highlights here.
According to Adobe, hardware acceleration is not supported under either Linux or Mac OS X, the latter because Mac OS X does not expose access to the required APIs.
Flash itself has to maintain backwards compatibility with old versions. Flash apps also tend to use a lot of CPU resources. It's a memory hog. The platform itself has a memory leak, which is why I earlier pointed out that a memory cleaner used in game does help. There are memory cleaner apps you can use that run in the background and work automatically.
Adobe Systems knew in 2009 that Flash had memory leak issues but had no idea on how to fix it and told that to The New York Times. http://pogue.blogs.nytimes.com/2009/07/02/why-do-adobe-flash-videos-slow-down/?_r=0
This article explains a lot. Great read, too, complete with Flash testing analysis for all browsers and operating systems. Flash Player: CPU Hog or Hot Tamale? It Depends.
http://www.streaminglearningcenter.com/articles/flash-player-cpu-hog-or-hot-tamale-it-depends-.html
Here's a couple of relevant quotes.
"Flash is efficient on platforms where it can access hardware acceleration and less efficient where it can't ... Apple complaining about Flash being a CPU Hog while not exposing "the appropriate hooks" to enable Adobe to access hardware acceleration seems disingenuous at best."
"... the ability to access hardware acceleration is the single most important factor in the overall CPU load. On Windows, where Flash can access hardware acceleration, the CPU requirements drop to negligible levels. It seems reasonable to assume that if the Flash Player could access GPU-based hardware acceleration on the Mac (or iPod/iPhone/iPad), the difference between the CPU required for HTML5 playback and Flash playback would be very much narrowed, if not eliminated."
So you're trying to play an AS game on WGT. Frank is running on his MAC, Sam has Windows 10, Jerry has Windows XP and Andy is running Linux. Just examples. What do you think is going to happen? Lol. See the problem?
Does any of this answer your questions?
I'm moderated on posts, so this is likely to appear hours later. Have a pleasant day.