Yes it is. It's -3 OoS, which, combined with G&W's frame 2 spotdodge, makes it unpunishable by characters without a frame 3 Up-B (or frame 4 grab, but I don't think those exist). This aint no Mario, "-13 on drop but you can sort of space it to make it safe sometimes" type deal. It is almost completely safe.
Safe on shield, perhaps, but not safe in general. If you miss it you'll suffer from some end lag (it ends on frame 25, FAF 39). Which, granted, is really low, and you'll likely won't eat any sort of hard punish for missing an up-smash, but it's still less safe than, say, using bair as Bayonetta or Cloud (partly because they get to move while G&W is stuck while up-smashing).
I think rage probably hurts Mr. Game & Watch more than many other characters. It's generally hard for him to live to high %, and due to his weight he can die stupidly early from rage opponents.
On a completely off-topic note, a glass baton would not struggle to kill you or be good at beating you up. If you tried to beat someone up with a glass baton, it would probably break, but then instead of a glass baton you have a sharp piece of glass to stab that person with.
Though you would still need to break it before you can kill, a glass cannon can just kill you outright. So it still fits, somewhat. I don't expect the term to catch on, anyway (and I doubt I'll use it myself, it'd be easier to just use words to describe his "easy to deal damage, dies easily, hard to kill" attributes.)