Nowadays the sites are largely demanding writers with simple writing techniques with focus on the grammatical issues properly. The ESL (English-As-A-Second-Language) writers though face some challenges yet they are capable of dealing with the modern day writing styles. There are great issues with the writing patterns nowadays. The writers with first language English may write in different writing patterns. However, with the ESL writers it is easy to deal with such issues. They strictly follow the instructions and form the articles purely based on the instructions.
The ESL writers are used to reading small pieces of articles that help them to understand the present writing styles and most importantly the format. There is some scope of creativity in the fiction-based writing but in the non-fiction, it is safe to go according to the guidelines without using sassy formats that cater to the risk of article rejection more often. However, in the initial stages they may face some issues being from a foreign language base. These writers take some time to getting used to the writing patterns but with their sense of commitment and dedication, they overcome the obstacles by the virtue of their hard work. Therefore, it is wise to hire some ESL writers to create an efficient team to deliver quality products.
You have rightly stated that the initial stages of writing job for the ESL writers are little difficult. But if they are sincer enough they can overcome such problems so easily.
I think that the biggest problem with these writers would be the writing style. Normally the writing style of the ESL writers would be simpler than the writers having English as a first language.. But what is important here for an article writer is that the contents should be interesting for the reader and the readers should relish the contents. And for this ESL writers are perfectly suitable as they would not try to write in any complicated style which makes it difficult for the reader to understand.
One more important thing is that many of the readers of the articles are also from ESL group and they would find it more suitable to read and understand articles of ESL writers.