Is Elphelt worth investing in?

I'm not positive if there's a general consensus on Elphelt yet. On paper, she seems to be a solid shotgun. She excels in both HP, and in armor. Her only major deficit is hilariously low evasion. Her buffs are good, but nothing special. Her skill is where things become more complicated. It's essentially a nuking skill on steroids, which I usually somewhat like/recommend, but her skill acts differently. Its initial cooldown is astronomically high, making it almost unusable in all circumstances except for BOSS battles. If you are like me and have a designated anti-BOSS echelon, she'd make an excellent addition. If not, keep in mind that she dummy links as a 5-Star doll. I try to keep my core stockpile above 350, so I personally have her at 3x right now. In conclusion, she is like IWS-2000 and NTW-20. She could really shine in some situations, but 90% of the time, you'd be better off with a high tier shotgun over her. (If you even have a Tier-1 shotgun, I know I don't) I'd say she's a niche-tier, but worthwhile if you think she'll fit for you.

As an EX doll, and having no duplicates, linking her is a pain in the backside. Her stats and aura are great, an accuracy and a damage buff make her fit perfectly into a solo SG echelon, with 2 HG and 2MG.
Her skill is kinda troublesome to use. With an initial CD of 10 seconds, well, you won't be seeing her using it that often. If she ever uses it, just grab some popcorn and see her nuke.
As I mention her stats are nice, high HP with a decent armor to fulfill her role as an SG, pretty good damage for an SG to Nuke.
Using her or not, depending on how you wanna play the game, I mean, she is pretty and isn't the point of Girls Frontline is to play with good-looking girls?

Tier-2 unit in the game, Tier-1 waifu in my heart

Yes, she is worth it. Her skill may be niche, but shotgun are usually useful due to their stats to begin with (And she has one of the best stats). I'd say she's even better than KSG as for now since we don't have that many levels where their skills are involved. She is definitely better than most Tier-1 Shotgun other than SAT as of right now in EN server. (KSG will soon outclass her in the future due to her useful skill though)

I terms of current content she's kind of average since you'll never really see her skill activate if you've got decent MGs in her squad. Currently her main advantage is a very quick reload for a SG.

Later on though she'll be much better since you're not going to be able to full field wipe every enemy unit before your MGs have to reload. That's where she's going to take off since her skill is excellent for cleaning up the stragglers.

With regards to tier lists though, it's best not to treat them as gospel but more an advisory. Look at more than one and you can start weeding out individual bias and look more into what units are highly ranked on more than one tier list. If one tier list recommends a unit that no-one else does, that's probably down to bias. If multiple tier lists recommend a unit, there's generally a good reason for it.