English is indeed complex in many respects —which of course native speakers are unaware of (just like native speakers of Russian don't see what's so difficult with noun declension).
A start is phonology; it varies based on dialect and analysis, but English has between twenty vowels and diphthong, in a system that is rather uncommon around the world; most languages are happy with just 5 or 7 vowels.
Likewise, English allows rather complex syllable structure, with very often two or three consonants in a row, and sometimes up to five ("Armstrong"), where many languages just build their syllables allong a strict consonant-vowel-consonant pattern.
Appart from that, English grammar is complex because the complexity that is lost with inflection is rendered with syntax instead. English word order is both strict and complex. Again, native speakers won't see what's the big deal (just as native speakers of Bantu languages don't see what's the problem with having a dozen distinct grammatical genders), but to foreign learner it's not that simple:
They have to learn to use definite and indefinite articles properly; many languages don't have articles at all, and those who do don't necessarily use them in ways that are perfectly equivalent to English usage (French requires the definite article before all country names unless the country is a city-state or an island-nation, for instance).
They have to learn the complex tense/aspect system of English.
English speakers take so much for granted that it's "she lives in New York" but "she's doing the dishes", which in the past tense turns to "she lived in New York" and "she has done the dishes" that they don't even realise that there might be something complex here to explain, and that not all languages do things this way (even a German speaker is completely at loss here, even though German has theorically the same tenses than English, but uses them in a quite different fashion).
They have to learn how relative clauses, comparative and superlative structures work —many language do these in ways that are very different from English, and even those who do it in relatively similar way might have trouble (French and English theorically use the same kind of structure for comparives and superlatives; except "aussi grand que"<>"plus grand que" is really not quite the same as "as tall as"<>"taller than" and it takes time to get used to!)
There the whole deal about "the man (that) I know" —even native speakers of English sometimes have argument about when it's ok to drop the "that" in this kind of sentence, so imagine foreign speakers trying to figure out this mess!
One thing that took a lot of time for me to get uses is that the 3rd person possessive adjectives of English (his/her/their) agree in natural gender with the possessor, whereas in French, they agree simultaneously in number with the possessor, but in grammatical gender and number with the possessee. So I sometimes want to write "his dog" because I know it's a male dog even though I know the dog's owner is actually a girl.
I imagine Spanish speakers have similar difficulties when they want to translate "su", which convey all of "his", "her", "its" and "their"'s meanings, except it still has to agree in number with the possessee!
On a more basic level, English question and negation syntax is very tricky for beginners, who have to learn which "is it?", "does it?" or "has it?" is the correct form for a given sentence, compounded with tense difficulties as well.
"Did he go to the store?" is not an intuitive way to turn "He went to the store" into a question for a Spanish or an Italian speaker, who would simply rather take the affirmative sentence and add a rising intonation at the end!
So no, there is nothing inherently simple about English, that's really just a native speaker bias to claim this.