It depends of how do you like to play.

Hiring mercs is fine as long you don't over do it.Downside is , with huge empire = money,you can attack just anyone by just sending one arme and 2-3 lone generals, which will buy mercs and get you additonal 2-3 stacks.

Having long suply lines , if you rely on homegrown troops,in huge empires is also fine and realistic.I found it for myself more intriguing , because it slows me down ,and requires more micro management, time and invasion/attack preplanning.But for someone that can be a downside.

Just play it as you feel is most enjoyable for you