There are two issues: Large. Reach.
In 5E, they have been reluctant to give PCs a large size. The large size makes dungeon navigation difficult, nd is actually a liability from the point of PCs being gang tackled (which is more impactful in 5E due to bounded accuracy). You can get a large size with a few techniques (includig the enlarge spell, wildshape, polymorph spell), but no creature, including the centaur, have a basic large size.
Reach is less of an issue. Bugbears have reach on their turn, they have reach weapons, etc...
When you combine the factors to create large reach (which I saw a lot with an Eldritch Knight that used the enlarge spell), you do not actually get that much benefit absent the polearm master feat and a polearm. There are wildshape/polymorph forms that are large (or huge) and have reach, but are not unbalancing, for example.
All in all, large reach is fine so long as it is not permanent. If it is, it may make the creature a bit hard to maneuver and a bit vulnerable to crowd damage.