The M5 is mainly an anti-vehicle unit and the other vehicles don't have the same health.
I honestly can't see how M5 is mainly anti vehicle with its current stats.
Puma stomps it and both axis factions have access to it, its 80 damage ensures even Kubel has a very high chance to just run away, its abilities were neutered, its penetration sure was buffed, but there was no light it couldn't penetrate and it most certainly is not viable against anything medium compared to cheaper AEC.
Agreed that M8 arriving too early would be a monster, but it arriving when it does now, you'd have to be extremely lucky to pull the value out of its current timing.