Even at high rank play the level differences are frustrating, because a lot of strategy and tactics assumes some basic ability to predict how units will interact - what trades with what, what counters what, etc.
A huge impact for example was that at level 7 Nod Bikes can kill an Orca in a single volley. Prior to that level that and the 3 bike missiles leave the Orca slightly alive (which leaves the Orca still a threat given how it works). If a Nod player is running a deck and relying solely on Bikes as their AA, I should be able to punish that with clever Orca usage. However, if they have higher level bikes then it basically becomes a hard counter.
Given the way the game works, when a level difference changes the balance between how units counter each other to this degree it's just frustrating. This is also very noticeable with how Bikes work against harvestors - the levels dictate whether it takes 3 or 4 volleys to kill a harvestor. This can decide games given how strong harvestor harassment is and how much getting an early 100 tiberium can snowball games.
In the latest patch most everybody is running very similar decks because Rhino and Buggy got overtuned along with Jackson/Oxanna. This results in a lot of mirror matches where players are countering each other with basically the same units - Bikes and Tanks vs. Bikes and Tanks - and it becomes noticeable how level differences affect the outcome.