1) You'll notice that the raider's building has bren carriers. That's for a good reason, cause you're supposed to mount them on it and raid.
2)They're not meant as replacement unit for commandos which is why they do not have the same abilities as commandos. The piat is to provide limited AT backup AND damage buildings
3)They're not replacement unit for tommies, just an addition of a close quarters non doctrinal smg unit. If you want more than that, choose commandos tree