I find Magnus' Provision Promotion is only situationally useful.
If my Capital has thumping production, then it can be useful so my Cap can pump out Settlers, keep Pop to work tiles, and so keep growing and pump out Settlers faster and faster. In that situation, Provision is particularly useful if my Cap has thumping production but low food, because then I really don't want to lose population because it's so hard to grow it.
But a lot of the time I really don't need to avoid losing Pop, because my Cap is still very productive bouncing between 2 and 3 Population. And if I have Magnus tied down in a City, I can't move him around chopping in Wonders etc. And of course, the promotion is completely useless if I'm not producing Settlers, e.g. because there's no room to expand, or all the expansion options are awful, or I'm stuck fighting wars.
Instead, I find Provision more useful when I'm settling later in the game, or can buy settlers with Gold or Faith, although settling mid-late game is a sure sign I'm not playing optimally.
Honestly, I think the Governors and Promotions are pretty well balanced. You really have to make tough choices around early promotions given how few governor titles you have, the promotions themselves are actually fairly situational (except maybe Magnus' chop, which is probably always useful, but I'm okay with that), and you often have to make tough choices about keeping a governor in one city or moving them around. Indeed, that's precisely why I very much dislike the free promotions in Secret Societies, because it really throws off that early finely tuned balance with Governors and early Governor titles.