Well, for playing games I use the flatpak version of steam and it works OK.
For dev work, it's great overall. Especially its ability to create separate reproducible environments with whatever dependencies you need for every project. However, there are some tools (rare, but they exist) that don't work well with it, and if your dev work happens to need them, it can becomes a problem.
For day to day (i.e. web browsing), it works the same as anything, with one disadvantage: there is a disadvantage here: it downloads a lot more than other distros on update, and uses more disk space. The biggest difference between NixOS, and say Arch, is not how it behaves once it's up and running, but in how you configure it. Specifically, you have to invest a lot of time to learn how, and set up your system initially. But then reinstalls, and (some of) the maintenance, become easier.
No. It's probably the best way to run packages from Arch, Debian. Ubuntu, Fedora, and others, all on the same system.
NixOS simply doesn't tackle that problem, so it doesn't come with containers out of the box. If you want to run packages from other distros on NixOS, you'd probably need to manually configure the containers.
I feel like you're under the impression that the three distros, NixSO, blendos, and Vanilla OS, have similar goals. I don't know about Vanilla OS, but the main similarity between the other two is that they're both non-standard in some way.
But they're actually solving completely different problems: BlendOS wants to be a blend of different OSes, NixOS wants to have a reproducible, declarative configuration (declarative here means, you don't list a bunch of steps to reach your system state, but instead declare what that state is).