Learn English – “By using”,”with use of” or just “using” (no preposition)

I would not refer to X as applicant. That is X' role vis-à-vis the people you are writing to, not a role in which you know X.

Instead, I would replace he at every third or fourth opportunity with X' name, particularly if I were eager for X to succeed in his application. Keep in mind that you are dealing with a correspondent who is probably dealing with dozens or scores of applications; anything you can do to ensure that X stands out and that his name is remembered when the applications are reviewed will increase his chance of success.

And don't worry about saying either he or his name too often. The whole letter is about him—it is perfectly natural for him to appear, in one guise or another, in every sentence.

Consider this answer more of a discussion than a direct answer to your question. The comments given above are interesting, but first of all let me summarize and then explain .

1) It is not a mistake to hyphenate "Comfortable-to-use" in the way you did, because it is a compound adjective.

2) Your grammar is right, but it does not convey the right meaning, because of poor choice of words and compound adjectives in your case.

Now for the explanation.

Point 1 :

It is not a mistake to hyphenate "Comfortable-to-use" - It is a compound adjective, so hyphenating it is correct. Consider other compound adjectives like state-of-the-art, mouth-watering, world-famous, these are widely used compound adjectives and most of the time they are not hyphenated even though they should be (here is an example). Consider the word "non-violent", you might have seen it being used as "nonviolent", yes it needs a hyphen between the word and the prefix but it is not wrong when the hyphen is ignored and the prefix just attached to the word, so naturally when you are exposed to a lot of words that originally had to be hyphenated but were not, using multiple hyphens as part of a compound adjective can feel weird and wrong, so I can understand why you were unsure about hyphenating "Comfortable-to-use". There is a lot of grammar behind it and I don't want to delve deeper. Usage of hyphen and apostrophe often confuses a lot of people and often misused as well, but in your case you were right to hyphenate "Comfortable-to-use".I always have 2 favorite examples to quote when I try to explain hyphenation

Examples : Man-eating(compound adjective)

1) Man-eating Crocodile vs Man eating Crocodile

2) Man eating Chicken vs Man-eating Chicken

The examples are a bit funny but shows you the impact of hyphenation.

Point 2:

Your grammar is right, but it does not convey the right meaning, because of poor choice of words and compound adjectives in your case. I say your grammar is right, because the way you constructed the sentence is correct, but still your sentence does not convey the right meaning because of poor choice of words. I am a software developer as well (C#, WPF) so I can understand what you are trying to convey, but the sentence falls short because of the use of "Comfortable-to-use". I agree with some of the comments to your question where they advise you to choose a single word such as convenient, or use compound adjectives like easy-to-use, I would say those are valid suggestions, not because "Comfortable-to-use" is not idiomatic, but because it does not fit the context.

As a software developer I always give top priority to reliability and ease-of-use of the software and I will make sure that the customers understand that as well. That is where your sentence falls short(by the way, falls short is an idiom not a compound adjective).

Comfortable vs Easy

Comfort is a physical experience, but ease is a quality. for example, lets say you are a English native but bilingual and you are equally good in English and French, and you are given a choice of writing a 10,000 word essay in either English or French, which option do you think will feel comfortable? It is not easy to write up a 10,000 word essay be it in English or French, but at-least writing it in English will make you feel more comfortable than writing it in French.

So when you talk about ease vs comfort in terms of software

Easy-to-use : Hey my software does not require much learning and/or practice to use it. It is intuitive and requires very less effort on the customers part to use it.

Comfortable-to-use : Hey my software can be easy or hard to use but it does not have any flashing images or a florescent green background, it is very comfortable to use but I cant tell you if it is easy or tough.

So replacing "Comfortable-to-use" with "Easy-to-use" will make more sense.

On a side note about idioms, idioms are basically group of words that cannot be interpreted literally or has a different meaning when it becomes a part of a sentence.

Examples:

Rub someone the wrong way - meaning to annoy or bother ,Jump the gun - would mean to be doing something early,Pay the piper - means you need to face the consequences of your actions, It's raining cats and dogs - It is raining heavily.

Easy-to-use literally means "easy to use", so "Comfortable-to-use" is just as much idiomatic as "easy-to-use" is, hence the reason it is idiomatic or not does not make "Comfortable-to-use" weird.