That's because the description of the M20 mine calls it an "M5" mine, which is complete nonsense. The M5 mine is the weaker AT mine Riflemen can put down doctrinally. The game game correctly calls the mine the Brits use an M6 mine, but claims that it is an improved M5 mine, which is again complete bogus. But hey, this is the company that introduced a unit called "Obersoldaten"...
Mines are highly situational and easily countered, if you lose a vehicle to a mine, you did something wrong, end of story. Unlike demo charges, mines stop working as soon as they are spotted by minesweepers, so even if you can't immediately disarm a mine, the mine doesn't pose a threat for the time being anymore - again, a demo charge still works, and since Pioneers have to basically sit on top of that thing to kill it, a demo charge is still a threat even when discovered.
And then you are also factually wrong about the Riegelmine, because it doesn't have enough damage to destroy the current version of the T-70 (320 damage vs. 400 health). That means even against light vehicles, you need either an additional Panzerfaust to kill them, or an AT gun - which means the mine works only slightly better against light vehicles than a Soviet TM-35, for a significantly higher cost.
Mines are not highly situational, as you can see in the second game of the gcs
https://www.youtube.com/watch?v=0RE2lL0Y-sM luvnest put a decent amount of schu mines, making the sweep of all of them almost imposible. You dont need to defuse the demo, you can kill it by clicking on attack move on them so you dont need to stay close to them.
And i didnt say that we need to remove mines, i just said that mines need to stop 1 shooting vehicles and inmobilizing. And yeah, the riegel doesnt 1 shot some of the light vehicles but it still inmobilizes and a bigger problem is the teller that 1 shots all the light vehicles and its non doctrinal, the m20 mine not only 1 shots light vehicles but also inmobilizes. Mines are a cool concept but i think all the factions need to have a non doctrinal mine that does the same
And also may i see your playercard please?