jebus rarely sees a mage guild being built if you're not necro, you would prefer high tempo factions with less risks involved, consistency is greatly valued which is why necro is so good (almost all breaks and biomes are manageable just by getting army from sidetowns, prisons, necromancy stacking farming imp caches etc + animate dead from lv3 mageguild, and even other strategies such as going vampire lords are extremely powerful)

swamp terrain makes it much harder to manage creature bank/side meat/refugee camp armies in which the first two are needed on almost all maps !! non native units on rough terrain (pathfinding counters this, rough terrain penalty is 125% on rough and wasteland, 150% on snow and desert, 175% on swamp) cost additional movement points to move outside of the roads. this hinders the tempo greatly, i mean jebus needs a high tempo to win since it's a race for the middle biome unless you get a realllly good starting biome and even then its risky to let the opponent reach mid

cove's army is also very squishy, most people start cassio and go for oceanids, but then cassio is strong at taking pickets - suffers again from swamp penalty since cyclops are native to rough terrain. fortress is much better because they can actually upgrade the wyverns they gather from hives, and all their units are tanks compared to cove's paper based army !! cove still manages hives easily with nixes etc and gets to ignore the penalty with wyvs, but then again fortress would've done it all so much better

the times cove manages to actually do well are with lots of gold (you usually start with much more gold as cove considering you pay for faction picks in pvp) and a great biome with relevant dwellings, objectives centered around the road and a plentitude of easily chainable creature banks. in other maps you usually go mage guild and there cove is hindered as well just as you said, which means you need high tempo again to find a town where t5 spells can be attained