Pinned straw:
New Hope is one of the better operators in the sector @PhilO, probably THE best operator, especially because they own their own infrastructure and have lower costs than most of their competitors, however it's risky to assume any dividend yield provided by a mining company (especially a thermal coal mining company) is sustainable, or even fairly reliable.
Take Yancoal as example 1:

Source: https://www.intelligentinvestor.com.au/shares/asx-yal/yancoal-australia-limited/dividends
Commsec has been playing silly buggers this evening and refuses to display dividend screens for most companies when I request them, so I've had to go with alternative sources, with the data above from II (as per the link) and below from the ASX website as well as from Commsec when Commsec was in the mood to play ball:

Dividends declining because both revenue and earnings are declining.


Next, let's look at Whitehaven (WHC) (Example 2):


Not a pretty picture; certainly very negative trends for both Earnings (EPS/Net profit) and ROE, and dividends will usually follow earnings down, and they are in these examples.
OK, what about NHC? Well...

Not AS bad, just FY 2024 (which is calendar 2024 for New Hope) where we see the drop off.
However dividends ARE declining with NHC also:

The 2025 half year dividend was up compared to 2024, but the overall trend remains steeply downward sloping. The dividends above are for 10 years (Commsec was happy to give me those, just not the same data for YAN or WHC) however the dividends below right are just the last 2 full years plus this year's 19 cps half year div, i.e. from 2023 onwards.

Maybe New Hope's future looks brighter than their recent past, but the historicals for all three coal companies don't look good, and specificallly their ROE, Earnings and Dividends are all trending down. There has been money to be made in bombed out sectors like coal and uranium in prior years, and there probably will be again, however the way I look at it, coal, and thermal coal in particular, is not well liked by many investors and so it is understandable that thermal coal companies are going to trade at discounts to their intrinsic value when there is less demand. It's like the opposite of a quality premium in a share price, it's the dirty coal discount. And NHC is mostly thermal coal.
I know the trend that Blackrock (the world's largest asset manager) started with banning coal companies from their discretionary portfolios and funds (but not their index ETFs) has lost steam and a number of fundies are now backtracking on their prior coal bans, and Trump's denial of climate change being negatively affected by what humans do on this planet is probably having an impact in that area. But I still reckon there's easier places to make money than in coal myself.
That said, I do hold shares in NRW (NWH) who have a number of met coal clients, so I have some indirect exposure to met coal through that picks and shovels play on coal (a mining services contractor: NWH), but no direct exposure to thermal coal at this stage, and that is not really based on any ethical stance on my part so much as it being a less liked (possibly "hated") sector where I don't see all of that negativity towards coal totally reversing any time soon, so it's just a sector with headwinds IMO. So not attractive to me as a sector to research and invest in. But each to their own.