It's only a cantrip, after all. It's not easy to design a cantrip that doesn't cross the line from "sometimes useful" into "I would use this all the time".
I, personally, would argue that 5e has done a pretty good job of making cantrips that are worth using without being stupidly broken. As an aside, I don't mind "use all the time" myself--the issue is whether a cantrip can, by itself, revolutionize a class or subclass without any extra investment and without the devs planning it to do so. E.g. it's fine that Eldritch Blast is powerful for a cantrip, because (a) it's designed specifically to improve the Warlock, (b) it really needs Invocation investment to fulfill its potential, and (c) it doesn't have any effects that could make the game go pear-shaped if applied to a different class (AFAIK, of course).
Hence why I said what I said. True Strike that applied to all attacks on the next round would be okay-ish for Wizards, but insane for Eldritch Knights, who can conceivably get 6 or even (at the highest levels) 8 attacks in "the next turn."
Just out of curiosity, would you use it more if it gave +5 to hit, so that it can stack with advantage?
Probably not? I mean, yes, that's a pretty substantial improvement, especially because it means that True Strike now has a very clear function: helping you hit things
you couldn't hit without it. So I guess that's your answer. I might use it more, because it has a clear case where it's (almost unequivocally) worth doing...but I'm not sure, particularly with Bounded Accuracy, that even that is "worth the investment." In the right party, knowing ahead of time that we will be
expected to min-max if we want to succeed, I might use it. That said, while I always give some thought to the "effectiveness" of a character I'm going to play, I'm not really sure I'd enjoy a game THAT heavily focused on optimization, so there's still not much chance that I'd use True Strike. Particularly when you can get 85-90% of the effectiveness just by having a single combat cantrip, and then filling all your other (very limited) slots with more "interesting" or "flavorful" or "utility" cantrips.
Twin Strike, even as a pure + to hit, doesn't just have to present some logically valid case for its use. If I'm gonna spend one of my, what, 2-4 total cantrips on it, it has to be worth the opportunity cost of
not taking one of the other cantrips.
Also, I'd be inclined to say that a first-level +5 to hit every other round is probably wonky; it wouldn't be such a big deal past level 8ish, when there are multiple attacks in play and proficiency bonuses start to approach similar values (~4). At first level, though, +5 is equivalent to proficiency AND a good stat, and if you already have both it means you're batting (essentially) 16 levels above weight. It'll settle out much like the Moon Druid does, I suspect, e.g. go from wonky to good to sub-par eventually, but for those first 3-4 levels in particular it would make Wizards, every other round, the best meleeists available, and I'm not sure I like that
In general, I agree with a sentiment expressed earlier in the thread. True Strike should've been an actual spell, and thus had enough power to be really worthwhile. Instead, it's a cantrip with highly situational uses either at the very highest levels (which most people won't reach), or only for the first level or two (which won't last long).