I take a mission and it give me Plus 2 for the hiring faction and -8 to the enemy. I'm making 4 time more enemies than friends. This can't be right.

If your mercenary reputation is more then a level or so above the recommended range for a conflict zone, it's normal that you only get +2 opinion per mission. You can move on to a higher leveled zone, you can accept the reduced opinion gain and write off some factions, or you can look into mods - one restores opinion gain even when overleveled.

If you do accept the reduced opinion gain, and have the DLC, I would recommend focusing on improving Davion, Steiner, and Liao, since these factions have questlines that have rare mechs guaranteed to appear (and are salvageable through normal mechanics). The Davion questline 'Bring her home' has a Marauder II 4a guaranteed on the final mission, the Steiner questline 'Stop the launch' also has a Marauder II 4a, but also has tier 5 UAC5s (which are ridiculously overpowered) on its final mission, and the Liao questline whose name I forget (the cultist one) has a lot of Lostech mech variants appearing throughout (lke a Black Knight 6 - the 6 and 6b happen to by my personal favorite mech chassis)

Marik, Kurita, Periphery Realms and the Pirates can all be fairly safely written off - they don't have little extras like three I mentioned earlier do. Whether or not Independents can be written off depends on if you're playing the story campaign or not - if you are, the last missions of the story have Lostech mechs appearing, while you work for Independents. But if you're just running a generic career, they too can be written off.

A final note - questlines give normal opinion boosts, regardless of mercenary level.

Thanks I have not modded the game yet