It's not like there aren't any MUs with more than 2 cars available. Hornby have done the 800/802/803, APT, 755 and a re-tooled VEP in recent years. Accurascale have announced the Irish 22000 in 3, 4 and 6 car variants. Bachmann have the CEP, BEP amongst others (am having some issues with their website atm but I'm sure there are more 🙂).
I daresay they might be perceived as more of a risk compared to a large steam loco or a popular diesel but they're pretty critical to anyone modelling a "modern" railway.
I suspect Accurascale might have something in development...