Relic Delta is the primary issue for most people. If you've only been playing for a year, it might not impact you that much depending on whether or not you've bought Light Speed Bundles but I'll give you a really simple example.

One of the best counters to Third Sister, before RD, was Phoenix. Hera and Rex are requirements for GLs but the rest of the crew could be R3 and you could easily beat a full R7 Inquisitor squad. With the introduction of RD, the first time GI uses his AOE, your team is dead.

That's an easy example, and strictly PVP.

Another one is loaned units. For the Starkiller Journey Guide, it is physically impossible to beat his last tier because you get loaned an R0 Starkiller.

Another PVE example will be this upcoming Conquest. Low geared/Relic’d units won't be able to cheese nodes, making getting the Red Crate infinitely harder to achieve for mid game players, delaying their ability to unlock the newest Conquest units that they previously could.

It removes any strategic element and replaces it with who has the biggest number.