Well I'll rise to the bait then (just once), partly because an outrageous statement has been made which should be challenged, and also responding to the invitation to show one's work for the benefit of others.
Let's start with a 'large boilered locomotive' shall we? I have featured it on here before - scratchbuilt 40 years ago by my dear departed model making 'mentor', Peter Sykes. Originally powered by a X04 and in many respects crude by modern standards (eg where's the brake gear?). But of considerable sentimental value.
Now rejuventaed with a DJH GB1 motor / gearbox. Could have used another option - yes - but, given that the loco itself cost me nothing, I decided to splash out, partly because the loco means so much to me but also so that I could try one out for myself (rather than comment about something I've never actually had any first-hand experience of). I'm still in the post-Mashima phase of trying out different options to see what works for me.
Now before Mick (and others) jump in and cry 'foul!' I will readily concede that the gearbox is only completely hidden by the fact that the builder has added a non-prototypical skirt around the rear of the boiler (between middle and front splashers). Without that, a small section of the gearbox would be visible (although some black paint would have largely 'sorted' that) - so I'll give you that one. However, and in case you're wondering about the slightly odd orientation of the motor, this was the only orientation that would work given that I was dealing with an already built loco. There's lots of structure at the top and rear of the firebox that I would have destroyed in order to get it in any other way. Weighing it up just now, I reckon that you might just get the thing in driving on the rear axle (with the motor pointing forward). I have recently acquired a Gibson 'Scot' kit off Tony so might one day put that theory to the test.
However, the REAL reason for showing this loco with its top off is ...
Given (Mick) your vociferous dislike of the product on the grounds of cost, you might not be aware - or have conveniently chosen not to mention? - that there are in fact TWO motor/gearbox combos in the DJH range, as I illustrate here. As I got on so well with the GB1 (not least because I now have a treasured loco added to the Shap / Carlisle stud), I've splashed out for a second time to see how the GB2 compares. As you can see, noticeably smaller (same price, mind - give or take!). I purchased it with the idea of this being the prime mover for the Millholme 'Patriot' I have under construction ... however, now I've seen and handled the thing I'm thinking it might actually be too small! But if your 'simply not suitable for anything other than large Boiler Locomotives' is at best contentious for the GB1 then it's simply blown out of the water for the GB2 ... as I shall now attempt to demonstrate.
Here's the current loco on the workbench, a PDK J20. This is a commission build (yes, despite my best efforts some folks seem to want me to build things for them).
Despite its large size (for a 0-6-0) - the J20 was fitted with the same boiler as the B12 and was the country's most powerful 0-6-0 until Bulleid's Q1 came along - it's tricky to completely hide the motor/gearbox, mainly because the firebox is set so far back of the middle wheelset.
Here's how I've gone about it (other solutions are no doubt available). To show no bias either way, what you see here is the very same Hi-level gearbox that I did a 'warts n all' assembly sequence on here a few month's ago. Mounted like so, with a squat Mashima (my customer had already purchased the motor/gearbox), it's a comfortable fit within the firebox.
Incidentally, I am in agreement with you Mick re hiding mechanisms. The kit designer obviously reckoned on folks driving on the middle axle so the pre-rolled boiler comes with a sizeable slot in the rear section of the boiler and no lower firebox front. As you can see here, I've added those pieces from scrap etch to complete the 'hiding' effect. Hope the customer appreciates it?(!)
Anyhow - it gives us an opportunity to compare. Could the GB2 be used in the same manner?
Well, I'll leave you to judge from these photos. My conclusion is that it probably could - just, especially if you regard the lower portion as being roughly where the firebox ashpan goes. However, the Hi-Level 'box undoutedly occupies less space so it's probably the better solution for this loco. But that doesn't make the GB2 'simply not suitable ...'
But now take a look at this. My 'Covid / lockdown' rapid build from last year. By no measure a 'big boiler locomotive'!
Here's what I did last year. Another Mashima (the kit had been in the build pile some years and when originally purchased, Mashimas were still freely available), married with LRM's own gearbox and extension piece, which allowed me to configure it so without any unsightly protrusions. I prefer to drive on the rear axle of 4-4-0s, in conjunction with a simple compensation beam on the front axle.
But would the GB2 have done the job?
Of course it would! Oodles of space.
Helped of course by the fact that the large one piece splasher hides anything below the boiler aft of the front driving wheel ... but no matter, it's a clear fit. In fact, Tony has made this kit himself and I think he might have used a GB2 himself?
It's only when you get down to REALLY small locos (you've already mentioned 'Y's, Mick) that the GB2 is never going to work. But we're down to the esoteric now - hardly 'many many locos'.
In fact, so puny is this thing that the whole kit is virtually designed around the rather contorted gearbox arrangement. As you will no doubt be aware, this is another Hi-Level product, the gearbox designed around this application. It was another beautiful piece of design and went together well, running smoothly and sweetly.
But, apart from things like that, I would regard the GB2 as a perfectly suitable prime mover for many small / medium size locos. Someone mentioned a J69 a little earlier and that's a good example of ample space inside many a simple whitemetal 0-6-0 tank engine body where there is no boiler envelope to worry about. Barry Ten has illustrated another example above
So - in conclusion - I find the statement 'simply not suitable for anything other than large Boiler Locomotives' at least as disingenuous towards the product as Tony's perhaps over promotion of them is the other way. From the comments above, there are clearly aspiring kit builders who are always looking on here for what options are available and - irrespective of cost considerations - to state that a product is NOT suitable when it clearly is - especially when that comes from someone who has not even used one (somewhat ironic when most of the pictures you post Mick are of large boiler locos (East Coast pacifics) that WOULD be ideal candidates for the GB1!) - is simply unfair to the manfacturer and their product.
Otherwise, hope that was of interest - maybe even useful? Now - back to Grantham preparations ...