Breakdown of Эти тренажёры не только современные, но и очень комфортные.
этот
this [f: эта, n: это]
очень
very
современный
modern
не только ..., но и ...
not only... but also...
Questions & Answers about Эти тренажёры не только современные, но и очень комфортные.
How strict is the placement of не только and но и in this sentence?
Very strict! In Russian, correlative conjunctions must sit immediately before the parallel items they connect. Here, they connect two adjectives. не только goes right before современные, and но и goes right before очень комфортные. If you moved the first part and said Не только эти тренажёры современные..., the meaning changes to 'Not only THESE exercise machines are modern (but other ones are too).'
Does the word тренажёры only refer to gym equipment?
It is the standard word for exercise machines like treadmills and ellipticals, but its base meaning is 'simulator' or 'training device'. You can also use it for a flight simulator (авиатренажёр) or an app that drills math problems. By the way, a gym is often called a тренажёрный зал.
Why are the adjectives современные and комфортные in their full forms here instead of the short forms (современны, комфортны)?
You could absolutely use the short forms here, as they act as the predicate of the sentence ('are modern', 'are comfortable'). However, in everyday Russian, full-form adjectives are very common for describing permanent, inherent characteristics. The short forms would sound a bit more formal or literary.
Is the comma before но и mandatory?