I've mentioned this in a few threads before - we've tested out many ad networks, but it seems like almost everyone only uses these three. Pirate sites don't have many options, so Adstera pays the highest CPM. However, many people complain that their site can be marked as malware by Google, and I've had a few sites that tanked a few years ago because of this issue. Therefore, I would advise staying away from Adstera.
On the other hand, Propeller pays well and on time, so we still use them. However, there are a few things that I don't like about them. For example, when you tell them that you don't want pornographic ads, they'll disable them at first, but then start showing them again after a few weeks or months. So, you need to repeat your request every time you notice them. Also, their response time can be slow, taking a few hours or even days. Another issue we had with Propeller is that after a few years of using them, they asked for account verification.
As for Galaksion, we've been working with them for three years now, and we've never had a single problem. The manager we have is awesome and responds almost immediately whenever we need some modifications. Payments are always on time and never late.
There are many other networks out there that are pure scams or operate like pyramids. Some of them may work well at first, but after some time, the CPM can drop drastically, even if you have the same amount of traffic. They will buy time by saying things like, "we have some problems, but from tomorrow, everything will be fine."
But let me reiterate, even though we may complain as much as we want, most people who use these ad networks have illegal sites or at least gray ones. So, we should be happy with the money we have made.