Risks are so minimal,drivers are out for main hardware and it can use Win8/8.1 in a few cases,stability well not had a single crash yet,remember Win10 has been in testing phase for quite awhile now,not last week or last month but quite a long time.
I would also say risks is virtually zero if you have everything backed up and nowadays there is no reason not too.
Again Win7 is far from perfect and on its way to EOL,don't kid yourself its better on security on 8 or 10 because that point has already be proved by Microsoft that 7 is not as secure,sure you can wait up to a year to upgrade to Win10 but why wait when it's very stable.
You have have more risks on the net surfing then upgrading to Windows 10.
I've just come out of three hours gaming session on 10 for a break, 10 has been awesome.
Some people have to moan about something,did not take long has I stated awhile back before 10 was released,don't like it go Linux...I never understand some people getting worked up on really minor issues like this,same goes for Aero and dozen things I could name.