So, having played through Monarch and bought the Lion, here's my review of the Monarch, the tier 8 RN tech tree BB. For my reviews of the KGV and Queen Elizabeth, links below.
Note: I am an average to a bit above average player who plays mostly solo, so this is from that perspective. Also, while I did consult numbers and stats, my reviews are primarily based on the feel of the ship and my performance in her. This is of course a limited sample size, and is affected by my playstyle (which is generally fairly aggressive leading to occasional overreaches and a low survival rate.) Your mileage may vary depending on your playstyle and skill level.
My review: [SCREAMS IN COLONIAL FURY] "WHYYYYYYYYYYYYYYYYYYYYYY"
I loved every ship in this line so far, so I guess I was overdue to hit the weak link that seems to be somewhere in every line, but really, WG? It's like you took tier 6 guns, put them on a tier 7 hull, buffed the reload to match the 14" guns on the KGV, and called it a day. REALLY? I have played every major line in the game up to t7, most up to t8 or 9, and this is the first time I have spent doubloons on free xp just to get out of a ship. Part of that was just pure frustration; I hit a bad 6-game losing streak where I had 4 of my best 5 games in Monarch (and my other two were at least ok) and lost every one of them due to unbearable potato teams. But part of it is...there is NOTHING fun about this ship IMO. She has zero carry capability even as top tier.
It reminds me of the transition from Konigsberg to Nurnberg as it used to be (I played through them in early 2016, so the meta was a bit different, to be fair.) Konigsberg is my favorite t5 cruiser. But Nurnberg is basically a Konig with some extra HP and a marginally quicker reload, only a tier higher and facing much tougher foes. So where K-berg was very comfortable and excelled, I struggled mightily with N-berg. Monarch is just like that. She couldn't take a punch; I routinely got LOLrekt. She couldn't deliver a knockout blow; where Queen Elizabeth's AP was decisive at tier 6, at tier 8 she just couldn't deliver damage consistently. Even with IFHE, HE wasn't punchy enough to be workable. My average damage dropped significantly in Monarch; while my average damage improved significantly as I learned to use KGV, it just stayed consistently bad in Monarch. The accuracy is still there; you just are too undergunned for t8-10 games.
At t8, you're facing three BBs with big, 16" rifles (NorCar, Amagi, and Alabama) and the German siblings with 15" rifles that handle much better and are paired with very strong secondary guns. Also, the Germans can afford to expose their rear guns; Monarch rarely can do so. You will not win a 1v1 in your Monarch with an even remotely skilled player in any same tier or above BB. Her AA is garbage, her speed and maneuvering are mediocre, and her secondaries are ineffective. Her stealth is outstanding, and this did save me a few times when I needed to pull back and heal, but let's face it, a BB isn't helping the team much if she's not firing her guns. Going dark for a minute or two to heal and/or reposition is one thing, but to keep that stealth, obviously you can't be firing. A BB that can only list stealth as a positive trait isn't going to succeed in most players' hands.
Basically, in transitioning from KGV to Monarch, you keep (roughly) the same armor and HP, about the same mediocre level olf AA relative to their respective tiers, about the same speed and handling, and roughly the same gunhandling and accuracy. In return for going up a tier, you get slightly bigger main guns (although you lose a barrel, with a 3x3 layout instead of KGV's 2x4, 1x2 layout) and better stealth. That's it. Yes, Lion also uses the same basic KGV hull and armor layout, but at least she gets the RN superheal and some more hp...Monarch has the regular repair party.
Monarch lacks versatility in most early and mid game scenarios; you are very limited to sitting back and HE spamming until the opposing team has been thinned out significantly. Every time I would push without a lot of help, I would die badly. Maybe others do better, but I am way too aggressive to succeed with a ship that is so unsuited for aggressive play.
Gunnery is just as accurate as accustomed. She still wrecks DDs and lower to same tier cruisers.
15" guns instead of 14" on KGV, but with the same traverse and ROF.
Speed is still ok, but for her tier it's nothing special.
She has OUTSTANDING stealth, allowing you to spring ugly surprises on occasion or back off and heal.
Feels very undergunned relative to her tier. Struggles to do damage against her peers or higher tier ships. She can still stack fires like her lower-tier cousins, but she has difficulty dealing direct, penetration damage consistently.
Melts quickly. Angling ineffective against most higher tier BBs. The only way to survive long enough to do damage is to hang back and let other ships tank, which makes it very hard to win if you have a timid team as you cannot lead a push or brawl in this ship.
Her AA envelope is small and, while reasonably strong, pretty much only suited to a battle of attrition with enemy CVs. They are going to get their drops off; at best, you might take out one plane from a squadron prior to their drop and hope to get a couple more before they're out of range.
TL;DR: Basically, they tried to take a KGV variant and make her a t8, but she is at best a t7.5. She wouldn't be a bad t7 at all. although she's a bit boring. But she's just not strong enough for tier 8. The increase in gun size isn't enough to compensate for the stronger ships she faces, and she gets the crap kicked out of her very easily. I think she's fixable, though. I think she'd be far less frustrating to play if she was a bit more survivable, so I'd give her a downtuned version of the RN superheal that Lion and Conqueror have; Lion has 7k more hp upgraded and a 1000hp per second heal; KGV heals roughly 250hp per second...Monarch should get a heal that is at least midway between those two. Also, while I admit the gunnery issue could be me and this is a small sample size, my gut feeling is they need to massage the guns and get her a bit more powerful. I'd suggest nudging the reload time down to 22 or 23 seconds, but it would seem really weird to go both up in gun size and down in reload time from KGV. I think it may be worth adjusting the penetration power somehow, or giving her the Lion's 406mms as an upgrade option (with, obviously, a lower ROF than her current 381mm guns.) That being said, her average damage on Warships Today is fine (and a lot higher than my personal average damage) so I think the survivability needs buffing more.
Final verdict: After two games in, I like Lion a lot more, and after trying Conqueror on the public test server she's also amazing, so I think I can safely say that Monarch is the low point of the RN BB line. Be prepared for a really crappy grind through this ship unless you feel like free-XPing your way through her.