Performance seems to be the biggest issue regarding html5 but people need to realize that its still fairly new, and if your an amateur programming regardless of the language you use (java,javascript,C,C#) your going to have a tough time scripting a whole game on its own with standard libraries. I have tried different frameworks with javascript and its much more effort then what C2 is capable of doing. For a cheap cost your saving a ton of time on debugging and your saving general development time. (I mainly use unity now but I find construct 2 fun to use, and still believe its practical for commercial purposes)
Majority of people who complain about performance are usually the ones that haven't considered the already available optimization methods while developing their game. Since html5 is fairly new technology and javascript is one of the slowest languages to use, you need to consider optimizing the game beforehand to compensate for these drawbacks, which is more then possible with Construct 2. Many can use C2 but haven't mastered it.
Someone experienced with programming would be able to create quick games in a matter of days, games that are runnable on most devices. If your a programmer, C2 allows you to execute javascript commands which is a plus. You can also create your own plugins for the game engine to add additional functionality. We even have wrappers such as cocoonjs,crosswalk and phonegap for mobile performance boosts. They all use a simple cloud compilation format so its even less effort to get games working on mobile devices.
Unity is a viable option but you need programming knowledge (javascript, C#), even then you need experience optimizing the games created using the engine to have it working on majority of devices.
The morale of the story is that a great game creator can use limited resources to build a decent product regardless of what their using as long as they have the knowledge to do so. This involves learning programming in general to debug on any tool, understanding image compression and such for optimization, and mastering a tool of your choice whether its C2 or unity.
That being said, I will list some personal drawbacks for construct 2:
1. The updates after R168 have a bit of issues on native browsers for older samsung devices (S2,S3,Galaxy nexus). This is bad for anyone that wants their games sponsored, but even so there are workarounds. But it would be nice if it was fixed.
2. The scirra team keeps posting templates of popular titles which ruins the fun out of creating the game yourself. When the flappy bird template came out, we had thousands of flappy bird clones. Although it's a marketing ploy it would probably make the engine more professional if we didn't have so many template options that kinda does everything for you. This is one of the main reasons why we have so many people that aren't able to master C2 and complain about it. Hopefully they make less templates of popular titles in the future.