As with all things free, you shouldn't expect the support of paid services. Since the Opera VPN services are available to many people, it may be a choke point. On top of that, your cell phone will need to handle encryption duties it doesn't normally handle. Here's the difference on my (admittedly underpowered) Nexus 5x.
Certainly serviceable for surfing on a cell phone, and would provide enough speed for most video streaming applications. But let's compare to speed with the VPN off.
The download speed is obviously much faster, but the real difference is in the upload speed, over 12 times faster! This has to do with network choke points, the processing needed to encrypt the information and possibly the limited RAM in my system.
One of the biggest concerns about the free VPNs is always a question of how do they make money. In some cases "free" VPNs are limited to a lower amount of data, hoping you'll pay for more once you hit the limit and are happy with the service. This is how Windscribe works (a system I tried for a while and found was rather speedy and had plenty of data for basic browsing, social media, etc).
Some insert advertising into your browser or as pop-ups. I still use the Opera browser on my phone because it drastically reduces data usage and blocks ads. Once every few dozen pages I get one ad, but by that point its blocked dozens, so I consider it a win.
The real concern are the ones who may be selling your traffic data to third parties. I suspect these are not common, but we don't know. What we do know is you wind up in the same situation you were in with your ISP; namely that one company knows about all of your online history.
If I had to give advice I'd say a safe browsing VPN from a known company like Opera is safer than open browsing on public wifi. Not a great solution but definitely the lesser of two evils. If however you plan on going VPN full time, its worth considering a paid solution. I've bit the bullet and use NordVPN, but that will be the next blog post.