The details in the first didn't gel together all that well as things developed - for instance, I never got why the homunculus in the shape of their mother was "Sloth"; nothing about her or her creation was slothful.
Actually, 'sloth' the sin has meanings that include: apathy/depression/neglect/laziness, and so on.
Therefore, Trisha Elric as Sloth makes sense cause she shows apathy towards her own children, neglecting her role as their mother and not showing caring/love towards them or indeed towards Wrath either. She later also reveals the other side of sloth, in that she did still care for her children, and yet was trying to kill them and not connect to her old life as trisha, purposely detaching from who she once was and her own memories/emotions. (a part of apathy)
She also appears to be the calmest of the homunculi, another representation of her apathy. (of all the sins, Sloth is the only one that really fits her role as Ed and Al's mother anyway... well I guess she could have been Wrath if she acted more angry and wanted revenge on the elric brothers for creating her.)
-----------
So that bit gelled Alright IMO, but I know what you mean for some of it not quite working as well as it did in Brotherhood.
On the flip side though it did make some connections (that brotherhood didn't) that I think were fairly cool, like homunculi being failed human transmutations, how truth was instead wrath, and was taking eds sacrificed body parts for himself, the gate leading to another world and revealing how equivilent exchange was an incomplete formula (Hohenheim saying how the energy for performing alchemy must also come from somewhere, and it was from the people dying in the other world beyond the gate. i remember think 'whoa, thats so true, i didn't even think of that before but it seems so obvious now that he mentions it'). Stuff like that.
I don't necessarily think its a bad thing that they weren't in Brotherhood, but at the same time they were pretty cool.