The Molly is VERY tough, with a massive structural integrity and biblical amounts of armour they are tougher than anything in service anywhere else save the Leviathan II. They are also fast for their size, being faster than most Clan ships save the York and Lola III (and some smaller vessels).
Armament wise they are built for close to mid range brawling and suuuuuuuck at long range engagements relying on luck with their few long range weapons to hit at ranges where you might find a Texas or McKenna sitting. Compared to Clan ships in theory they outgun anything smaller and basically anything save the Potemkin and McKenna, but they are built with different roles in mind.
A Texas and McKenna are both designed to fight at long range, using their big gun batteries to get nice bracket firing bonuses before popping away at extreme range. And at those ranges the Molly simply can't reliably reply save with its missiles or the odd lucky hit with the heavy and Medium PPC's or heavy guass rifles. With them at best you can get a maximum of -1 to hit, whilst the Clan ships will be getting -3 or -4 to hit and in space battles thats HUGE.
Up close where they can bring their NAC-40's to bare the Molly can easily outgun a Texas but you've got to get into range where you can use them accurately as you've only got a limited number of shots so you can't just go DAKKA DAKKA! and hope for hits, with 13/12 roungs per gun you NEED to be sure of the hit.
The Molly has some defences against fighters with her ER large laser and LRM batteries but these are really little more use than deterrence, her main defence will be her 32 fighters and 4 dropships. But here you get a problem.
The Molly has an absolutely TINY cargo capacity, just over 14k tons, and whilst in a pick up game this basically means bugger all, in a campaign this is a HUGE drawback. This means that the Mjolnir has an absolutely pathetic range as you'll burn through your food and water VERY quickly with a fully loaded ship (including the crews of her dropships). She'd literally have to go *jump* ATTACK SOMEWHERE *come home again*. Or what you do is give her say two cargo dropships and load them up with food, water, spare parts etc if you want her to go a little more than one or two jumps before having 3/4 of the crew die of scurvy. Also there's only 1 (large) grav-deck making life uncomfortable on long missions. This all points to a ship that was designed to go out, punch someone in the throat and then call it a night and go home. Not patrol the borders.
And if you take some cargo dropships (which you would need to do in a campaign) then you've only got two escorting dropships and for the love of Blake don't put a carrier dropship with her as that will just gobble up her supplies even quicker.
There's also no shuttles or small craft so you'd probably have to fit some of those which would reduce your aerospace fighter complement as well.
The way I look at the Mjolnir is she's a big strong body builder of a vessel, but she has no endurance. If you can get close with her, you'll delete anything that gets in her way save a Leviathan II as you can take a pounding on the approach, but if she's unable to close she can't do much. But if you can get into a brawl with a hostile ship or two, then they are in trouble, and the Molly with her thick hide, can take a real battering before thinking about withdrawing and her heavy guns at short range can smash opposing ships into kindling.
As for Strengths and weaknesses
Strengths
Absurdly strong hull and armour
brual firepower up close
Fast for a ship her size.
Adequate fighter compliment
Weaknesses
weak at long range
No AMS or point defence weaponry (which would become an issue in the Jihad when EVERYONE save the Clans was lobbing nukes around in space battles)
Absolutely no edurance without affecting her dropship complement
Light anti-fighter defences that are of better use of scaring off a fighter strike rather than defeating it.
She's still superior to pretty much anything in House service, and would do well against other House warships from the 3057 or 3067 book as they are all built with the same kind of focus on close in fighting, this also applies to most Clan ships although something like a well handled Black Lion could be an issue and the Texas, Potemkin and McKenna are far more nasty, and if there's a Leviathan II or Blake help you a Leviathan III, time to close your eyes and be prepared for a lot of stinging. Cpmpaired to the big ships of the other Houses at the time the one she'd not want to face is the Thera. That many fighters will drown her and disable her, but everything else, as long as she gets in range she can remove them with ease. The Feng Huan is woefully undergunned (the Feng Huan II cruiser carrier though...) Against an Avalon its a case of not being too badly damaged by missile hits on the approach before engaging at close quarters, the Agamemnon is laughably fragile and a few hits could destroy one.
A nightmare for this thing to face would be a Kimagure class. Its both faster and has lots of very long ranged guns with decent bracketing numbers. In a pure gun on gun battle, if the Kimagure can dictate the range (and with her higher thrust rating she pretty much shall) the Mjolnir would have issues hitting her whilst the Kimagure would have to wear down the Molly's armour.