Is Drupal good for ecommerce websites ?

I think BuiltWith will do this for you. Plug in the domain and it will tell you which technologies it is built on. Most of the well-known video sites are custom jobs, but I'm sure WP could do what you want.

I've been down this road before (two custom CMS platforms built over the years), and to play devil's advocate here: do you really need to build a custom CMS? What problem are you up against that aren't being solved by existing solutions? If those problems aren't fundamental, how much time and money will be saved by creating plugins for an existing CMS? I know the temptation to start from scratch, but if this is something to be used in production, the todo list is staggering: you'll need user authentication, security on the front and back end, admin interfaces, plugin infrastructure for extensibility, theming (if this will be used for multiple clients), third party integrations — and that's all before you get into really annoying and notoriously finicky shit like media uploads and i18n. If it's for learning or a hobby, go nuts, but if you're planning to use this with clients, save yourself the heartache and late nights and use something established and open source. Join the community and help fix existing solutions' shortcomings — that's a bigger overall contribution to the world. Good luck!

For the design? The downside is that you could end up with a very generic looking application/site. It's also not your (or your developer's) code so there will usually be a period of time required to get used to it in order to make any adjustments. In the end, it may not really save you that much time...Depending on what it does or doesn't do. Templates certainly aren't a bad place to start though. They can save time in many cases. Just not all cases.