If I remember right (don't have codex in hand) Maulers don't have enough attacks and there are too many other Hvy support units I'd rather have. I've been using a unit of spawn with mark of nurgle in a fast attack slot for grins.
Heavy support contains most of our firepower- Forgefiends (who can say no to two hades autocannons or 3 plasma cannons?), Defilers, Obliterators, Havocs, etc. To use up one of those slots for an assault walker with only 2 attacks and ws 3 when the entire rest of our codex is full of assault units can be.... sub-optimal.
If I felt I absolutely needed an assault walker I would take a helbrute or defiler instead- the helbrute only takes up an elites slot, which is no big loss for us (or even comes without force org from formations), and the defiler can be an effective assault combatant without sacrificing too much firepower, and has more attacks and hull points than the maulerfiend.
Some people prefer Obliterators, because Obliterators can deep strike and are more durable, but personally I feel that Obliterators are only worth using if you make use of the deep strike to multi-melta things at short range, and even then its pretty risky to deep strike such a large points investment as an obliterator squad. If I am going to sit back and shoot things, I prefer havocs. They are cheaper and don't stand out as an obvious threat when viewed by your opponent in the way that oblits do.
I haven't field tested either unit yet, but am planning on running Havocs in both a 5 man squad with autocannons and a 5 man squad in rhino with meltaguns for tank busting shenanigans. Lascannons I feel are just overall inferior weapons when compared to the combination of Autocannons and Meltaguns. (Lascannons are only really superior to autocannons if the enemy is packing really well armoured tanks, which is only really the case with Guard, Necrons and Marines, in which cases Melta weapons take over and the autocannons just pop transports)
Still, a forgefiend is probably better than Havocs & Obliterators, but a lot of it comes down to personal preference and the independent needs of your army.
I usually run 2 Obliterator units and a Havoc sq with 4 MLs with Flak for my Hvy support choices. Oblits can change weapons to fit the threat, have two wounds, and can move and shoot. I give them Mark of Nurgle to keep them from getting insta killed by lascannons and meltas. I don't usually deep strike them. I have them move across the board providing fire support to my infantry wave.
I'd rather run Spawn next to my Raptors instead of a 'mauler because of it's low number of attacks.