Showtime’s duo of not-so-blissful domesticity, “Nurse Jackie” and “United States of Tara,” return with new seasons on March 28.

Showtimelogo_lowry Both shows have garnered their share of acclaim, deservedly so in regard to stars Edie Falco and Toni Collette. But the two series also have significant shortcomings that are very much on display — and in the case of “Tara,” magnified — by these new episodic flights.

Tara_tall At first, “Tara” was a cute, almost wacky premise: A mom with dissociative identity disorder (DID), or multiple personalities, creating an unusual set of headaches for her husband (John Corbett) and kids (Keir Gilchrist, Brie Larson). The touch was light, and a little bittersweet. Hey, mom’s nutty — somebody else even — but we still love her.

Since then, however, the program has grown darker, and veered farther from its origins. This latest season adds wrinkles that border on slasher territory, introducing a dangerous new “alter” as well as a college professor (Eddie Izzard) who is both skeptical of and fascinated by Tara’s condition.

In short, the new batch of “Tara” episodes — despite Collette’s abundant charms — simply aren’t much fun. They also get bogged down in peripheral characters, particularly the relationship between Tara’s sister Charmaine (Rosemarie DeWitt) and her awkward baby daddy Neil (Patton Oswalt). As good as Izzard can be, moreover, these episodes don’t make much use of him.

Created by Diablo Cody, the show has the look of a concept that didn’t know what to do for an encore. And while I’d like to see Tara finally face down the issues that caused her to adopt alternate personalities, the prospect of sitting through a fourth season to achieve that closure is hardly appetizing after finishing all 12 episodes of the third.

Nursejackie_lowry “Nurse Jackie,” by contrast, has never been a laugh riot. Indeed, Falco herself seemed mystified when she won the Emmy for best actress in a comedy, protesting that she isn’t funny — and the character, certainly, isn’t.

The show nevertheless won fans as what amounts to a half-hour drama, drawing heavily from the talented New York stage community for key guest roles.

Jackie, of course, has her own problems — a stressful job where she’s the voice of sobriety, a husband (Dominic Fumusa) and kids, an addiction to pills, and a needy boyfriend who (Paul Schulze) who won’t let go.

Those worlds collide, rather amusingly, in the season premiere. Unlike “Tara,” “Jackie” also sticks to its knitting, for the most part — the problem being that not all the threads (to pummel a metaphor) are equally interesting. “Jackie” would be considerably better if the supporting players popped, but I have a hard time caring about anything that isn’t directly related to Falco. And some of the quirkier twists — like a push at the hospital to get Michelle Obama’s attention by becoming childhood obesity advocates — feel too precious.

In short, Showtime’s dysfunctional moms are both testimonials to the appeal of a strong central character — and otherwise, illustrate different brands of creative dysfunction.

There’s some symmetry in that, perhaps, but as TV experiences go, it amounts to under-achieving.