One could say that Ultraman and Kamen Rider (or whichever series where the heroes don't make an effort to conceal their identities) are produced by different companies and leave it at that, but I find I don't like this explanation. It is possible (and I've read something along this line) that the Ultra hosts fear repercussions from all the planes/buildings they have destroyed/crashed, but is that really the reason? Kamen Riders break things too when they fight, but I don't see the townspeople getting upset at them. As for hiding their identities for fear of the enemies capturing people dear to them as hostages, I don't really see the point in that either (in the heroes believing so, not in the villains refusing to fight dirty), and never really feel it is the main reason anyway. Not to mention those aliens in Ultra series always know who the Ultras are, even if other humans don't.
If anything, maybe the Ultra hosts are even a little selfish for keeping their secrets. (At least, that is mostly what other characters seem to accuse them of after learning their secret identities. These people seem to say something along the line of the heroes taking away their (other characters') choices, making decisions for them.) My question remains: if it seems to hinder more than help other characters, why do the Ultra hosts feel the need to be secretive about their identities? That is, apart from inspiring the requisite dramatic, 'oh god, this guy is a hero, why can't you see it, don't you have eyes,' type comments which the viewers feel totally justified in shouting at the clueless characters.
I could probably attempt to guess at Mebius' motivations a little at least. He is not human, so he would want to fit in, wouldn't want to be seen as different or weird. I don't know if there are prejudices among the Ultras, but to be singled out couldn't be a pleasant experience, whether one knows what prejudices are or not. And he didn't yet know these humans. He wouldn't have any idea how the knowledge of his identity would be received. For all he knows, humans don't care much for aliens (see: episode 7).
In the end, however, it is probably nothing more than tradition. Since the first Ultraman doesn't reveal his identity, subsequent Ultras who come after him don't either. Not that I really like this explanation, but it seems the best I can come up with at the moment.
Since I have no good answer for the secret identities question, I want to talk more about Mebius and how the series seems to deal with the secret identity issue. First off, Mirai is Mebius, and the viewers know this since Day 1. My question as I started watching was, would someone else realize it over the course of the series? Given what happened in the past Ultra series, I had no hesitation saying yes, someone would. In fact, I would be very disappointed if no such revelation occurs. So, the real question is not if, but when someone will find out his secret.
This may sound like bragging (oh well, I admit it is bragging), but I was certain by the end of the first episode that Sakomizu knew exactly who Mirai was (I just didn't know how or why at the time). This was confirmed (in my way of thinking anyway) in episode 4, which also revealed one more person (Misaki) who knew Mirai's secret.
Fast forward to the middle of the series, Mirai started to run out of creative excuses (admittedly, maybe they were not all that creative) for the time he needed to disappear to fight as Mebius. I took it as a signal that his time of dodging the truth was nearly over. And it was. The confession/revelation scene arrived soon after.
I'd read that some people thought Mebius might be over at that point, since the hero's identity was revealed. This is probably me bragging again, but I never believed the series would end there or soon after at all. I sort of understand the reasoning for that line of thinking, however, since most previous Ultra series save such revelation of hero's identity toward the end. Thing is, most previous Ultra series also use human hosts, with "human" being the operative word here. For each of these series, even with the secret being out in the open, the dynamics among/within the group would not really be changed, since the host is still a human, mostly making future episodes repetitive and/or the same as the ones before the revelation. Not having much new material to work with later on is most likely the reason such revelation takes place near the end of the series.
The above scenario, of course, does not apply to Mebius. True, the group didn't or tried not to treat Mirai differently from the time before they knew who he was, but subconsciously, they probably did act toward him a little differently in some way. Mirai is an alien, and normal humans probably couldn't help feeling in awe and/or feeling curious about him. Anyway, this is my roundabout way of saying that Mirai and GUYS' post-revelation interactions are just as interesting, if not more so, than their pre-revelation interactions, therefore the series can definitely go on for another twenty episodes without (for the most part) boring the viewers.