Just like after OF was released everybody was fascinated by the eels. Everyone used to auto lvls 1-3 with 8 eels. They were a great unit for their time but after other parts came out, their strength decreased. So we can't categorize them as "imbalanced" before we see the artillery and fast units.
Pretty much - Ronin might still stay top dog for the age, but that'll be the fault of other units primarily if so. As things stand:
Ronin beat all previous age units other than turturrets fairly well. This isn't necessarily a problem as they're previous age units and there's any number of units you could say this about in reference to what units came before them (AAVs, Stealth Tanks, Hover Tanks to name a few).
Ronin beat the two other currently introduced VF units relatively well, but those other units also have properties that just make them hard to use - even against earlier age units.
However:
The fast unit is not likely to be something that poses a threat to the Ronin as it has bonuses vs fast. It's in fact hard to imagine a fast unit that samurai are going to be able to use their contact with range 1 against that that range 1 doesn't also just get them slaughtered by Ronin + Rogues while doing no damage.
And after the artillery unit comes out Ronin life is either going to get even easier or maybe stay the same - they'll be avoiding the same fights they're avoiding now with a lot of artillery at best.
If they continue the 10 unit approach from OF, the second Light unit might be the first opportunity to see a less awkward non-artillery unit that counters them.
There is however precedent for ages where one army winds up being the answer to pretty much everything : CE and the AAV+7 rogue army. And Ronin aren't quite THAT strong in the current scheme ( i was at least losing some units testing out qaccy's assertion of just autoing with ronin through 3 difficulties - i don't lose any usually autoing through the first 3 difficulties with AAV+7 rogues in CE ).
I'd also suggest that nerfing the Ronin so that we have 3 useless units instead of 1 good and 2 useless can't be the answer as we'll just use earlier units instead then - already I'd for instance consider the Scimitar a better Samurai for most uses - sure it dies in two hits to many things, but the samurai took one of its 3 hits from retaliation and only hits one target at a time.
Edit: Perhaps what we'll see is a Fast unit with Blast? That would make sense as one of the things they counter is Ninjas who do have hide, and Blast causes the AI to try to melee things it shouldn't so that'd make the contact on a range 1 unit (occasionally/rarely) useful.
Edit 2: And then if the fast is Blast, perhaps the artillery could be stealth on plains to gain an edge in the artillery vs artillery match against the earlier age giving an age where the durable Ronin has trouble hitting things?