flash always crashed my ie 11 on my windows 8.1 machine ,I compared the cpu usage from task manager and the results were :- when the flash was turned on cpu usage was at 88-95% with ram almost 70% used (just playing a video from YouTube in ie 11) , when I disabled it and turned on html5 from YouTube settings the results were :- cpu usage 35-40% , ram usage 40% , flash player is a resource hog and windows update keeps on throwing flash layer updates every month .
conclusion :- I don't want a buggy , outdated/not supported feature on my windows phone , even apple (ios) does not flash player , they discontinued it in 2009 I believe.