Welcome to the fold!

The only things you really have missed are event units from previous events because you started out late. However, there will supposedly be revival events later so you can get them, one way or another. And the event units are really only missed if you cannot complete the event. That said, events usually have something that low level players can do to get the event units. I started right in the middle of the Dragon Shaman event and i still managed to get her since i could clear a few of the maps after power leveling. The only difference being that i could not get the other objectives like reducing her deployment cost and increasing her skill level.
The other events seem to have followed something similar. You can usually get the units and they are nice to have, but you cannot get them at their best unless you are high level. Not something to cry over, though, they are still awesome