1) Fabricated Steel
The pros of fabricated steel housing are that it is very strong and that it can be repaired and welded on. Unfortunately, the material and process makes it pricier than cast options. Fabricated steel housings are used on modified or customized gear drives.
In all cases, there is a chance that a poorly made housing will leak oil through the welds or pores in the housing. Quality manufacturers will pressure test their housings to ensure that leak paths do not exist.
If you are concerned about the housing dimensional fit or construction, work with your supplier. They should able to provide you with outline drawings or models during the quoting phase. Quality requirements for housings, such as inspections, should be communicated to the supplier through the PO.