I never made any mods in WC3, but the tools are definitely better than the ones for the Source 1 engine. It is much easier to work with models and materials, add animations, etc. However, if you want to modify existing Dota 2 models, you still have to decompile them using 3rd party tools. In my opinion, that was one of the biggest issues with Source 1 modding... You had to rely on all of these fragmented, broken and outdated third-party tools and plug-ins. Hopefully Source 2's tool chain will get better -- not worse -- with time.
One of the biggest issues right now is simply a lack of documentation. The community is doing a pretty good job of figuring things out, especially the people at moddota.com, but the process involves a lot of trial and error.
I also find the particle editor to be pretty confusing. I have a lot of trouble creating my own particles from scratch. Many of the operators and initializers have no description, so again, work involves a lot of trial and error. I usually just resort to taking an existing Dota particle and changing a few colors or adding some control points.
I'm sure things will improve as we gain experience and Valve enhances the tools/API, but right now it is a bit of a struggle.