Honestly pretty solid. It does sound a little weird, but it's an adult Milhouse so it fits. It almost sounds like they just did a regular Milhouse voice and then changed it to age him up.

Mo Collins does an excellent Jimbo Jones.

Alex Desert took a little time to figure out Carl, but it's pretty solid now.

Kevin Michael Richardson still does not do a good Doctor Hibbert, it's a little too... Not quite Cleveland from Family Guy, but Family Guy background voice character. Which is a shame since he's talented, but maybe the issue is that they seem to have changed the character and made him less serious.

Which I only mention since Megan Mullally took over the voice of Sarah Wiggum a few seasons ago and made her a wildly different character. But in the case of Mrs. Wiggum she didn't have much established story.

In general it seems like they're doing a great job at replacing voice actors with new talent. I do however fear the day we lose one of the big ones. Julie Kavner's voice has changes the most noticably, although she seems to be doing better as this season has progressed. So hopefully they've figured out a way to either adjust her voice or she's figured out a way to be more comfortable when acting.

Also since I'm ranting way outside the question I bet Harry Shearer, as the oldest and grumpiest, retires next, and that's going to be tricky.