I'll agree with a lot of users here that HTML5 is a massive headache, the cross platform mobility is great, but we don't even really have that for anything more advanced than a simple game.
to be fair, the only thing we do not have I can think of really is a direct control on ressources.also both tools are different, unity is meant to do full fledged 3D games natively first, whereas C2 is meant to do complex 2D games that can run inside a browser and so, overcome a lot of basic distribution issues (there is no visible risk of a virus for the average user, the security installements will not prevent it to run by default, you have one version that runs everywhere unless you used wrappers, with relatively hi performances for a browser based tech) at a cost of third party relying (mainly browser vendors that can be a big trouble sometimes with thrir custom interpretations of specifications).
basically, both tools are nice for their respective usages and have issues based on their philosophy, which you should already know if you bought them.