There have always been a debate about stability and functionality of video games in online multiplayers. Initially, video games were meant for some quality time alone but that's not true anymore. Majority of Gamers who buy original copy of a game are more likely to play it online with friends, crews or even strangers as pirated version generally kick off campaign or story mode without
any major issues.
1. Lag
With most server located in US or generally developed countries, the chances are you gonna have really bad ping times if you are in any other part of the world (especially 3rd world countries).
2. Aimbots
This has been a rampant issue just like piracy but in completely different form. Even with Valve's Anti Cheat or VAC secured servers, there are plenty cheater who use these tricks without getting banned.
3. Cheats and Hacks
Just like Aimbots but generally in non-shooting games such as Minecraft, Sims etc. These are really annoying if you're a pro at a game yet failing miserably because of those dirty little bastards.
4. Difficulty
As a casual gamer, like many. I always struggled in MMOs and competitive online games such as Team Fortress 2, Counter Strike Global Offensive etc. Many gamers are fine with single player mode but when it comes to competitive online gaming, it can be a challenge for lots of us.
5. Mods
This might not be as negatively effective as the ones mentioned above but it does cause heavy stability issue with the person using different mod or original game.