Probe spam and RCS spam are what makes people blind to the actual true instability of their shuttles. Instead of engineering the imbalance out, probes and RCS gets added as a shortcut to artificial stability. But in fact there is no stability, it's just an eye blind. Probes add 3 tons per probe which reduces your delta-v and load-carrying capacity quite rapidly. RCS saps fuel like nobody's business due to its very low ISP. And in the quantities commonly used for "balance", the amount of fuel wasted is really significant. That results in a sharp decrease in delta v. These two methods of so-called "balancing" a shuttle are nothing but poor engineering and a dead loss.