The only people who should be concerned about installing iOS 10 when it is released in fall are those with older hardware or jailbroken devices. The lastest jailbreak tool only supports 64-bit devices running iOS 9.3.3. iPad and iPhone owners with jailbroken devices will need to avoid iOS 10 if they want to keep their jailbreak tweaks and apps installed.
iPad users with older devices, such as the iPad mini 2 or fourth-generation iPad, may want to hold off on updating to iOS 10. Even though the iPad 2 and iPhone 4s both support iOS 9, the devices can't take advantage of most of the new features. There were also reports that iOS 9 made the iPhone 4s and iPad sluggish, and there were a variety of bugs and issues with some of the later iOS 9 updates. However, Apple normally does a great job of quickly fixing all major and minor problems, like it did with the link crashing bug in iOS 9.3. There just wasn't much use to installing iOS 9.x on your iPhone 4s and iPad 2 and it did allegedly cause some users headaches.
Apple has been testing iOS 10 with both developers and the general public for a few months. It is safe to assume that all the bugs and performance issues will be worked out before it is finally released in the fall. However, iOS 10 does not offer a lot of iPad-specific features, and it does drastically change some of the layout of your device, such as removing the Slide to unlock feature. If you're married to the current look and feel of your iPad or iPhone, then you may want to stick with iOS 9.3.3 for as long as possible.
iOS is mostly about new messaging features. It does offer a new side-by-side Mail option for the iPad and a new camera UI, but that's about it. The messaging features, such as sending confetti and adding stickers, is fun for about 5-minutes before it gets annoying and you wish there was an easy way to disable them. There are also a few new Photo app features, the ability to delete some stock apps and a few other tweaks and improvements.
iOS 10 should be safe and worry free to install on later devices such as the iPad Pro, iPad mini 4 and even the iPad Air 2. Not to mention, it is always good to update when possible because most updates include performance and stability improvements. If you're on the fence about updating, and don't care about missing out on all the new features, then there is no reason not to stick with iOS 9.3.3.. I do have to remind you that sticking with iOS 9.3.3. means you will be missing out on message effects and all the new emojis!