I haven't heard enough of him but I know what you mean, somebody can speak very good English but when they have a flat accent (like some spaniards) it can be so hard to follow. Shows how important the sing song nature of a reguonal/national accent is to the language itself.
I also think a contributor factor is the way Spanish structure their sentences, which can often lead to difficulty in following them
Most people, who learn a second language, try to translate from their native language into the second language, in this instance from Spanish into English which can result in vagaries. The best linguists, can, automatically think in the second language and are totally fluent.
A simple example would be, in a sentence constructed in English we, normally, would use an adjective (or verb) first to describe the object of the sentence a noun e.g. the “red carâ€, in Spanish the noun precedes the adjective and becomes the “car redâ€
Personally, I am able to grasp, without too much difficulty, Carlos’s thread when he speaks.