I’m not a mod, but this is a strategy forum, not a balance discussion. There’s a time and place and I just want suggestions for gameplay in the current patch, not what should or shouldn’t be done about the next one. Read the OP and if you’d like to contribute something useful go right ahead, and if you just wanna complain about how much usf sucks head over to the balance forums. I might even agree with you, but that’s not what I’m looking for here.
Then don't feed trolls, I asked you what was your BO for the Lieutenant start, which is one of the most important information if you need some help. Still waiting.
M20 is too costly today, yes you can build one to harass and protect your flanks but with the recent 222 price reduction, you're simply calling for a 444 vs Ostheer and since 80% of OKW are going T2 vs USF to build a luch, you're most likely going to spend your time repairing it every moment than anything else.
M15, personally I'd rather not build it if I can. Yes it is a mobile suppressive platform but that's all since MrSmith decided to cut his dps in half for no other reason than OKW HT was overperforming at that time and get nerfed so to make sure the M15 wouldn't ended too good. The unit dies in two AT shots, once a Pz4 is fielded its days are counted because of its clumsy mecanism and low health profile.
I'll add that I watch games and cast and yet can't really see a player making a strategy around them.
So in conclusion, yes you can make use of them, I don't say "don't build them or don't use them", but they are completely situational, you can't make a strategy around them. M20 is there to counter sniper and M15 if you need an average armored vehicle to keep 222 or luch at distance with a squad of zook nearby.
Any other case, better stale for fast sherman, HE and punish as much as you can lone squads.