Right, and people have won against release day Brits, doesn't mean its not a skewed matchup.
And yes, kubel spam. You say "fast" m20, but against kubel spam, the enemy should have such a large fuel advantage that their luchs comes out only slightly later than your m20, if not at the same time. Nevermind the fact that the luchs just countered your m20, but you also have nothing that can even damage the luchs except AT nades and weapon rack upgrades because you were forced into lieutenant (and, again, using zooks against a luchs is just asking to get wiped with all of the red cover).
tl;dr: the m20 is hardly a counter, and the cover availability on the capture points on the road is the issue
I actually have never had problem vs kubel spam as USF. Kubels wont scale well into mid game, and luchs is managable. What i would suggest u is to make second RE, dont spread out your main army too much, and rush some rifleman to key covers, (and let 2nd Re cover other side of map), and rember always to focus fire sturms first (and anythig else whats coming), kubels will die eventualy, when your rifles will reach vet1, and will be upgraded. If u are in godo position, just take captain.
The key is not to get into bloodrage, and mindlessly chase shit around map, use USF long range to your advatnage - Priest doc is my choice, and u can use M1911 LMG to your favor, to rip schreds on any axis infranty, or u can take armour, and rape him later, cause, all these open fields mean - no chokepoints, so your m10 horde can roam pretty easly.
If u go with M 20 - dont overextend, make mines (try to bait luchs on em), and always keep vet 1 rifleman close, make AA HT later, and get yoru zook squad out, it will be enough to manage 1 side of map till 8cp (if luchs dont die on mines).
But best way to learn, is - just take okw, try to do kubel spam, and watch how others crush it, then just do the same as usf.