Corrosion in the gas system was certainly a factor for those who preferred recoil operated MGs. The Colt really didn't have much of a problem in that area, because its gas exited into the air, not into a cylinder. Nonetheless, the Hotchkiss and its derivative, the US Model 1909, the Marlin, the Lewis, and the BAR all preceded the BREN, and all were pretty good guns. The BREN would have been a better gun if it had not been changed to use the gawdawful .303.
In general, the idea was that heavy MGs would be belt fed and used in fixed positions, while "light" MGs (not even the BAR is light if you have to carry it a while) would be used in maneuver units.
The Johnson LMGs were OK (I like the side mounted magazine), but the M1 is far superior to the Johnson rifle in almost any category.