Never seen one in person I an really partial to npk they just run 3 of mine are as old as i am just rebuild and keep going. I have ran a few epiroc witch bought out atlas they seem decent Stryker okada and even cat hammers as well as bti are ones I would be willing to spend money on mainly cause I have a little experience with them
Cat hammers are not all that great in my opinion and I dont think okada is much better Stryker doesnt build a component based design it's more unibody witch is really limited to rebuild same with cat
Let me add a little bit of (what little) knowledge, and personal experience, I have to this.
I agree that Cat hammers have, in my opinion, not been my first choice in the past. I believe that they used to be made by Rammer and the way they were designed caused them to destroy themselves and were expensive to repair/rebuild. They would hit pretty hard though. Cat has recently changed their hammer design (I don't know if now made by Cat or who?) so I cant speak to their reliability or performance. CM1995 currently has one and I'm sure he can comment from a more informed perspective.
Striker (the hammer that I currently own) is made by Toku. This is a Japanese brand that has been making hammers since the late 40's. Striker is how they are branded in the US and they are also sold under the Komatsu brand world-wide. Toku used to make BTI hammers and, again-in my opinion, how BTI made a name for itself in the hammer market. BTI hammers are now junk (in my opinion). BTI was bought by a large holding company and now sources their hammers from a Korean MFR (I don't know specifically who). If one can find an old TB series BTI they are made by Toku and can be serviced/supported well worldwide. My breaker is a 5,500# energy class breaker (smaller of the "large" size breakers) which I bought new in 2017. It is a TNB190 and it is comparable to NPK's GH-12 breaker. It probably has about 400-500 hours on it now and the only thing I've had go "wrong" with it is one (1) bolt came loose on a cover plate recently. I've gone through a few hoses, and a couple of Stucchi couplers, and have just installed my 3rd tool, but NO mechanical issues thus far (YAY!). I also have a friend who runs 7,500# and 10,000# Toku hammers (started out with Cat hammers) and he reports that they are extremely durable, and are relatively easy and inexpensive to rebuild. Since I have not been inside one yet so I can't confirm or deny whether, AzIron's comments (regarding "component based design") are correct. When I compared NPK to Toku (during my purchase research) they seemed to be very similar in design. Though each MFR has a slightly different design they seem to employ the same components. Both have replaceable upper and lower bushings, both have anti-dry fire mechanisms, both employ only (2) moving parts, both use a nitrogen cell to recycle energy, etc. Toku does employ a (2) pin tool retainer system vs. NPK's single. If there is a difference in which components are on them I am not aware of it. I think both are good hammers. AzIron has stated (in other threads) that they use (and prefer) the older NPKs based on performance, ease of rebuild, inexpensive initial investment (many older NPKs are out there in the used market) and simplicity. If there are some differences between the older NPKs and newer, maybe he could shed some light on them. I did narrow my hammer choices down to Toku and NPK but went with Toku due to my friends experiences. In addition to those we had a mutual acquaintance who "bare" rented several wheeled excavators with Toku breakers (this was in the 80's and 90's so they were older BTI branded-made by Toku-units) and he reported that he had almost no repairs on those hammers. I would be afraid to let anyone else run my breaker let alone send it out for any "Tom, Dick, or Harry" to run unsupervised. Those unsolicited comments/experiences spoke volumes to me. NPK makes fine hammers and I would not likely hesitate to own one. There are a ton of them out there and the only negative thing that I have heard about them are that they can have a lot of "pulse" energy (surging in the hydraulic lines with each hammer blow) and some reports of the the bore moving inside the housing. As it relates to the other brands I don't have much experience. I do also have a breaker for my mini (a Bobcat brand 950) which, I believe is a Stanley breaker and it has been pretty reliable for me (bought it used in +/- '08). HEF member KSSS claims to love Okadas but I don't know what size units he is running. Since you will likely rent it may not matter which brand you go with as you will not likely use it long enough for any issue to arise from a sub-standard brand.
I mentioned size in several of my comments because size is critical for breaking rock. Breaking concrete is EASY when compared to breaking rock (particularly in situ bedrock). Concrete is usually about a 3,000 psi material, and some types rock can be over 20,000 psi. Obviously the energy it would take to break concrete is substantially less than that of rock. Keep that in mind when you go to the local rental store and think about renting a hammer mounted to a mini-ex or a rubber-tired backhoe. Those will likely be 750-1000# (on the high end) hammers and there is rock out there that will tell my 5,500# breaker to "F"-off.
That last statement reminded me of how good Striker's support is. I first bought a TNB110 (3,000# breaker) and found that it would NOT break most of the rock I was encountering at my sites. I showed some video of it working (or rather trying to work) to the president of Toku America at the '16 Con-Expo. Based on that he sent me (2) brand new-on the pallet breakers (the next 2 sizes up) for me to demo at my jobs -at no cost to me. I chose the larger one and they gave me back my full purchase price for the TNB110 as credit towards the TNB190 that I have now. For me that demonstrated HUGE integrity and commitment to customer satisfaction!
The last thing I would say to you is relative to your last post. Whether you buy a Krupp, or any other brand, be sure to "but it right". If buying used quality rebuilt units can be found at specialized "hammer" repair facilities/dealers (at least here in the US). Auctions are good place buy hammers (at massive discounts) but the term "BUYER BEWARE" applies here. If I were to buy at an auction I would be sure to pay a price that still allows me to do some serious repairs/rebuild to it (should it turn out to require it) and to cover the hassle of that process as well.
Good luck!