I recently bought a Netgear powerline ethernet adapter that was capable of 300Mbps data transfer and should (theoretically) push my full 105/12 Mbps Comcast service. I actually get roughly 35-40Mbps, which sucks to have to settle for that kind of loss, but the reality is that you're hard-wired with that speed. Barring any major electrical noise, your latency will be super-low and the signal will be consistent. My ISP's .ac router has the range to reach where I'm at, with fair signal, but there's too much signal power loss (best case scenario = ~55dB... OKAY at best). I ran into intermittent signal loss that would screw with my web browsing and my online gaming at times - super annoying!
Jumping to a powerline adapter greatly improved my connection, even at the loss of speed. To get more to the point, the more stuff you have plugged into your walls across the house, the more opportunity for signal loss due to the noise on of those currents. If you have a ton of stuff that you leave on for no good reason, get in the habit of turning them off. I found that my high speed USB phone charger for my Nexus 6 and my Xbox One were creating a lot of noise that greatly degraded the signal from 40 to as low as 5! Now, I usually can leave my Xbox One on, but power cycling it ramped up my speed instantly, so I knew that was one item that (if left on for many many hours at a time) is capable of bottle-necking my powerline adapter.
As stated earlier, even older wiring can support good speeds, but remember that your wiring can run hundreds of meters depending on how everything is setup, and jumping between circuits further drops your signal - enough to probably cut your bandwidth by more than 50%. So if you don't have a fast internet service (under 15 Mbps), powerline adapters could ultimately be pointless, since you could see subpar connectivity. However, newer homes with up-to-date wiring can prove to offer lower noise when using these adapters. My home isn't new... not old, but not new. A newer wiring setup would probably double my speeds.
In the end, if you're choosing between accepting weak WiFi signal and decent, if reduced, powerline speeds, choose the later for sure. If you'd rather, run a powerline adapter from your source and put it maybe mid-way between your modem/router and where you need internet then hook up an access point. You'll probably pull the best speeds that way (if you aren't required to have a hardline connection). It just costs a bit more since you need the adapters AND the AP for that.
