Oh I don't know. The last two episodes do throw out a lot of "oh mysterious!" crap with the inner workings of Big Fire, but I don't remember them being hard to follow. Introducing plenty of characters with little payoff because they are clearly for the next chapter that never happen isn't a great idea, but the core story is concluded and pretty great.
To me, Giant Robo is a masterpiece. One of the few anime that you could really say is an epic in scale. I thought the story is really well told and pretty layerd for something called Giant Robo. Then again I really like and relate to father son themes and the main conflict is all about that. It does have some really cool characters though and yeah the look of the show is ace.
But yeah I also watched it years after it was completed.