I overlooked the discussion tab and will remove all but the original comment & place them here in post. The comments section probably is not the place to drive a point
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In response to: “And it's (the rate of mapgen corrupt sites) supposed to be significant - otherwise what's the point ?”:
The point is allowing counterplay to the Necromancer's unique ability to "salt the earth" through corruption of income sites, & to a lesser extent create a unique beneficial alternative to the Well of Souls.

Otherwise the main mod benefit is allowing a broader range of valid targets for Corrupt the Source as a number of sites were left out making the thematic spreading-death theme of the necromancer feel a bit thin.

Changing between 25% & 30% of income sites to be useless for the breathing-class majority is a major balance shift and a disproportionate one at that due to uneven distribution. While the cleanse source might not necessarily make a large impact on gameplay without a larger number of already-corrupt sites; corrupting a significant portion of sites on mapgen represents a balance shift in favor of necromancers and a general obstacle non-necromancers. In the last two campaigns running the mod I’ve encountered corrupt great farms within the first 3 cities acquired and have not uncovered cleanse the source for the majority of the campaign; 50+ turns. Researching all the avatar spells to maximize the chance of uncovering cleanse just isn’t strategically worth it. The net effect of default corrupt sites is less population growth for non-necromancers.

Even if my experience is a statistical anomaly and uncovering cleanse is as common and quick as implied; losing out on 100+ population growth per turn in the early game represents a significant decrease in trajectory of non-necromancers. The mod’s choice of default corrupt sites begs the question: Why is the shift away from default exploitable sites necessary?

If the concern is cleanse lacking applicability without common obstacles for use; looking at cleansed versions of Hall of the Forefathers & Castle of the Lichking would add desired pointedness to the cleanse addition and be more in line with a late research acquisition.

P.S: My removed Comments for posterity.

Jul 29 @ 4:56pm
The mod adds needed functionality to the corrupt/cleanse metagame that can be very onesided but the increased amount of corrupt site generation that occurs with default map generation seems pretty excessive.

Jul 30 @ 11:14am
I can't speak to the ratio without uncovering an entire map. I have, however, seen occasion where both nearest independent cities have corrupt greatfarms and such useless substitutions to the 6/7 breathing classes when cleanse is typically uncovered so late is a rather significant change to the vanilla mapgen settings for income structures. I'm not really sure why such a balance shift is good or needed.

The whole climate metagame & similar corrupt/cleanse should be available on a more general basis, at least in a less efficient means than specialized magic; due to the potential extremes that could arise from certain picks that might not allow one player any recourse against certain actions. This mod adds that needed functionality but by altering the existing mapgen balance of income sites it makes a significant alteration and I'm not sure why that should be.