I do agree that humans' actions affect this earth, not necessarily in good ways. (Earth getting hotter is the first example that came to mind.) But the universe? I laugh. True, I don't have any idea how vast is the universe, save that it is probably close to infinite. Still, saying humans will destroy the universe seems to me like saying one small insignificant ant will destroy Earth.
And the show mentions so much scientific stuff (even though mostly made-up, I believe) it makes my head spin. That one time-warping episode still leaves me confused, no matter how many times I've seen it.
Gaia is not my absolute favorite series, but I think it's the best Heisei Ultra series in terms of storylines.(2) It does not have anything that seriously annoys me (sadly, other shows do). And I've never heard of anyone who had negative opinions of this show, or who just thought it was so-so. (Now, will someone prove me wrong, I wonder?) Well, maybe scientific people might find much amiss with the theories passed around casually in this show, but since I watch any toku show as an entertainment, I don't let those details bother me. And Ultraman Hikari fans? You should see the original blue Ultraman who started it all.
(1) This might be based on an incorrect translation (from episode 47), so my apologies if I spread misinformation.
(2) As a disclaimer, I do like all of the following series, and yes, a few more than I like Gaia, but...
I think Nexus is too dark to appeal to kids, and, in many ways, it is not a very traditional Ultra series. It is very possible that people who like Nexus may not enjoy any other Ultra series.
As for Seven X, my reasons are the same as the ones I give for Nexus. Also, the show is too short to develop its characters properly.
Mebius' main strength is not plot, imho, but characters. Mebius' characters have the chance to grow because the episodes tend to focus on them a lot. I don't know. I kind of get the feeling that if one doesn't care for GUYS, one would have a difficult time liking Mebius. Seriously, one can't even hang around for the past Ultras alone because they take ages to show up. (Outside of the first movie, I mean. ^^;)
Tiga is an all around good show, but, curiously, the main character (who is close to nauseatingly pure) doesn't seem very well developed, as compared to other characters. He seems like an old school type of hero, who knows his duty and does it without complaining (well, apart for one time or two). I believe I understand the reason for his lack of character development, but I'm not sure how to articulate it into words. Perhaps he just seems almost larger than life. He doesn't seem to have any discernible failing. Almost every mistake I've recalled, he does not cause it. Most of the time, it's someone else.
As for the episodes, they are perhaps a little too independent of one another. (Yet, besides a few clearly marked ones, Tiga does have some interconnected episodes that are not very obvious, for example, episodes 15 and 16 / episodes 37, 38 and 39. I admit I didn't come up with these connections myself, but they make senses to me.) But what is an Ultraman show if not episodic? As much as I prefer them, story arcs don't seem very typical of Ultra series.
As for Dyna, it has a different kind of main character problem. He is a comic relief, to put it simply. And rather bothersome, actually. Dyna makes fun of itself too much for me to take the series very seriously, but it does have a good side. Still, it's not really a series I would choose to show anyone who has never seen an Ultra series before. (But I still think the Miji seijins episodes are ones of the best. These guys are just too hilarious and pathetic in their repeatedly failed attempts to conquer Earth. :)
Cosmos is another series with the main character problem. I'm not sure sometimes if he is Ultraman Cosmos. Not because I don't believe him a hero, but because Cosmos and he seem too much separated entities. And he is annoying, in a way like a child who needs growing up. And if one can somehow overlook him, one needs to look to team EYES' idealistic goals of saving/protecting monsters. Yes, the environmental concerns are also (actually, first, since Gaia was made before Cosmos) covered in Gaia, but the way they handle things in Gaia seems more grounded in reality (well, as much reality as a fictional show with giant monsters can).