It's been a while and I confess I never acting got the reforms in game (too many CTDs), but browsing the EDU and previews there did seem to be some nuanced changes between the Camillan and Polybian units.

One is with the triarii - as has been said, they go from being phalanx to spearmen; they also lose their breastplate and AFAIK are a little "downgraded" in stats (Qwerty is tweaking them to be less godlike). My feeling is that in RTW in general, phalanxes are great infantry killers - at least against the AI. You can line them up and crush non-phalanxes with impunity. So the early triarii are a good counter to infantry elites like gestatae (ok, it would take forever to kill them, but they are very good for pinning them to allow javelins and cavalry charges to the back). Perhaps perversely, I don't find phalanxes to be very good against cavalry - they are too slow to catch them and, in vanilla, they have the distressing habit of becoming disordered and switching to swords (maybe different in EB if early triarii lack swords). By contrast, in RTW, I think spears are often inferior against infantry but - in the right conditions - good against cavalry. So, I would be tempted to use late triari in a counter-cavalry role and less gung-ho with them against infantry.

The other change I noted was the principes lose their spears. For the reason given earlier, I think this would mean they get rather better against infantry and rather worse against cavalry. That probably does not affect gameplay too much, though, as the late ones are still beefy enough to take a cavalry charge while likewise the early ones are a match for most infantry.

I am not sure when the velites appear - if they replace the levees with the reform, then that would be a rear powering up of Roman skirmishers in terms of their melee potential.