I just mean balance-wise
Their spearmanii are worse than the germanic equivalent after the germs get their first upgrade, but roman ones are much more expensive
Their cavalry are in every category pound-for-pound worse than equivalent cavalry in virtually any other culture. They don't have proper spear-wielding heavy cavalry, meaning they're at a huge disadvantage in cav vs cav battles and their shock cavalry are genuinely the worst in the game. But once again, are more expensive than the better options in other factions.
They have no armour piercing melee options, so they don't have good ways to deal with other factions' heavy infantry
And their ranged options are very limited; only paper-thin t1 archers that melt to counter fire and crossbows, which are good but horribly outranged by bows. They have no good way to deal with heavy-ranged factions.
A general pattern in their roster is that their units are pound-for-pound worse than everyone else's but also really, really expensive relative to what other factions pay and they're lacking a lot of important options in favour of just having a tonne of redundant defensive infantry.
The only real upside of their roster are crossbows, Scorpions, Large Onagers (they get access to them a whole research-tier early than non-roman factions) and Federati Spears, which are genuinely a good unit (and probably the only exclusively WRE unit that stands out as being actually good).
For the most part, every standard part of the Roman Army gets an elite endgame upgrade in the ERE, which lets them pull off the whole 'defensive line' faction better than the WRE, and they have a massively better cavalry roster, elite pikes, dane axes and elite bows so they just generally have a more flexible and balanced roster while having almost all of the same advantages as WRE (eg. early access to Large Onagers). Plus the ERE's passive eco boosts greatly offset the higher cost of some of their shared units.