I've exclusively used test enanthate for over a decade, with no pip or other problems. I've heard others complain plenty though, so people do have issues with it. The recent test E I've used has come from Opti and Stan, and both have been pain free.
This is likely to be largely meaningless in the grand scheme of things, but one reason I prefer enanthate over cypionate is because it's a simpler ester. Cypionate has a cyclopentyl ring, whereas enanthate is a simpler straight seven carbon chain. There was a recent study showing cypionate to have significantly lower peak serum levels when compared to enanthate. Theoretically, phosphodiesterase 7b, the cleaving enzyme that is responsible for removing the ester, has difficulty dealing with the bulky cyclopentyl ring present in cypionate. This applies to esters like phenylpropionate as well, which is commonly present in NPP and testosterone phenylpropionate, a test ester found in Sustanon.
This is testosterone enanthate:
View attachment 339092
And testosterone cypionate:
View attachment 339093
The three dimensional images give a bit more insight.
Here is testosterone enanthate:
View attachment 339094
And testosterone cypionate:
View attachment 339095
I could be wrong about all of this, so feel free to correct me. These theories were based on just a single study as far as I know, but the reasoning made sense to me. If enanthate causes too much pip to use, obviously none of this matters, but I thought it interesting enough to share.