I did the upgrade from Win 7 Pro to Win 10 Pro on an 8 year old netbook (1.6Ghz N270 Atom cpu with 1.5GB of RAM) I had laying around that wasn't being used. The upgrade went fine and everything is working without issue.
A lot of the purported security issues with Win 10 are only active when you use a Microsoft account. I use a local account to login with so things like web-sense aren't active.
I don't like Win 10 enough to bother upgrading any of my other machines but this 1 machine that I upgraded is running great, haven't found any issues yet.
MS claims over 14 million computers have been upgraded so far. I have no idea how many have subsequently rolled back to the previous OS but I'm inclined to think the percentage is small but even if it's 10%, using MS claim of 14 million upgrades, that's 1.4 million that rolled back!
If you like Win 10 but had problems with the upgrade, you can download the
MS MediaCreationTool which will let you create an ISO or thumb drive to do another install. Once you have done the upgrade at least once, you can now do a clean install of the OS (wipe the drive and install Win 10 on a clean drive). This is the preferred way to install a new OS anyway, upgrades have historically always been problematic. If you do the clean install, when prompted for a key, skip that step, MS will still activate the install because you did the upgrade. You can't do the clean install until AFTER you did an upgrade.
A clean install will wipe everything off your drive so make sure you back up your data and have the installers available for all your programs that will need to be re-installed.