What is it like working as a human resource specialist in an Engineer Battalion S1?

Pretty much the same as every other battalion. S1 shops typically split the duties up among the lower enlisted, one person will handle awards, another will handle something else. I've been in every type of unit except Cav and they all work this way. Reach out to your sponsor though, they can give you more details.

I actually did this, once upon a time.
The biggest difference was that there was a ton of manpower in the organization compared to others. My HHC in the engineers was bigger than my whole Civil Affairs Battalion is now.

Same like working on other S1s except you might have better accomodations since you will be stationed in an engineer battalion

About the same as any other Battalion S1 Shop. Combat Engineer Battalions are just a bit bigger that an Infantry Battalion. If they are fully staffed, the work load for the junior enlisted is usually divided up between awards, pay and promotion, and general Admin stuff. If they are doing their jobs, it's pretty busy.
It's not the most exciting job in the world, but if the S1 section is screwed up, there are lots of unhappy soldiers. I was Active or Active Reserve for a bit over 13 years, less than 2 as S1, but at least 70 percent of the questions and problems that are brought up here go through the S1/G1 section.

I've been in Infantry, MI, Cav, and EN units. S1 is S1. The ONLY exception I saw was my brief deployment working with the SOF community (I always caveat, I am mot and have never been SF, I was just supporting them). Even then, S1 was S1, they were just BETTER at it.

You pretty much do the same things as anywhere else, but keep in mind that since you fall under an Engineer Unit, you'll have to meet the ACFT standard for Engineers even though you're a 42A...Engineers among a few other MOSs have the highest and hardest ACFT level.