As many have said, its going to be campaign dependent, and DM dependent whether or not int is useful in a given game. That said, I think that mechanically, int is very underwhelming. Really, as long as you have someone in the party with proficiency in investigation and probably arcana, you're going to be in fairly good shape, even if they took a 10 in int.
As far as planning strategy, I'm not sure why int would necessarily be the go to. A fighter with a soldier background probably has a lot more practical experience in small group tactics and could reasonably offer a lot of good suggestions in how to approach a fight, regardless of their int score. If the party is trying to break into a building undetected, a rogue is going to be the person leading the planning, again, their int score can reasonably take a back seat to their training and experience.
This got me to thinking, is it that int (and str) are underpowered, or is it that wis and dex are overpowered? Maybe the issue is having too many things depend on your dex score, and having too many wisdom and dex saves, and passive perception being such a big deal. That if there was a way to knock those down a peg or two, then things like int and str would seem more useful. Oddly, con only really gives you two things, hp and con saves, but its still generally an important stat for every class and build.
Characters with higher charisma than the player are also hard ... for the DM. Often the player will make a not very convincing persuasion, and the DM forgets to allow a persuasion roll, and not increase the DC for their poor rhetoric.
For characters with higher intelligence than players, you can ask for intelligence roles to get a hint at the puzzle. The opposite I find harder. I play a +0 Int character, but I singlehandedly decrypted a message without the secret key. I probably shouldn't have been able to do that.
As many have said, its going to be campaign dependent, and DM dependent whether or not int is useful in a given game. That said, I think that mechanically, int is very underwhelming. Really, as long as you have someone in the party with proficiency in investigation and probably arcana, you're going to be in fairly good shape, even if they took a 10 in int.
As far as planning strategy, I'm not sure why int would necessarily be the go to. A fighter with a soldier background probably has a lot more practical experience in small group tactics and could reasonably offer a lot of good suggestions in how to approach a fight, regardless of their int score. If the party is trying to break into a building undetected, a rogue is going to be the person leading the planning, again, their int score can reasonably take a back seat to their training and experience.
This got me to thinking, is it that int (and str) are underpowered, or is it that wis and dex are overpowered? Maybe the issue is having too many things depend on your dex score, and having too many wisdom and dex saves, and passive perception being such a big deal. That if there was a way to knock those down a peg or two, then things like int and str would seem more useful. Oddly, con only really gives you two things, hp and con saves, but its still generally an important stat for every class and build.
Characters with higher charisma than the player are also hard ... for the DM. Often the player will make a not very convincing persuasion, and the DM forgets to allow a persuasion roll, and not increase the DC for their poor rhetoric.
For characters with higher intelligence than players, you can ask for intelligence roles to get a hint at the puzzle. The opposite I find harder. I play a +0 Int character, but I singlehandedly decrypted a message without the secret key. I probably shouldn't have been able to do that.
Very important, if your DM rules that disarming a trap is an INT (Thief's Tools) roll.