Having dealt with a few of these options, and having hosted Holdfast myself both through a service and my own hardware, I'll give my unbiased opinion on the three providers we used. I'll rate them from 1 to 10 based on hardware (H) and customer support (C). I'll add a brief description of why as well.
As for my credentials, I'm a network admin and network engineer in real life, and I was the highest ranking active leader of the late 89e Les Enfants Perdus regiment.
H(2) --- Not once could I use all of my slots on an Oasis server and have a smooth game. Not. Once. There was a point when I was tired of it and made a bit of a stink, after which they moved us to their new server (which I was told I was the only one on), and for a time I came to within 50 users of my actual slot cap before the server gave up the ghost. But that didn't last as apparently their new server died, we were moved back to the old server, and it was back to using half the slots or less.
C(7) --- Oasis was reasonably responsive when pinged about server issues, however the main technician is a one-man-show so to speak. Whereas gamers never sleep (meaning someone is always using a server), this one individual certainly had to sleep eventually. When they were awake, however, top quality support.
Warlance Game Servers
H(7) --- Still could not use the servers at max cap (the slots I paid for); but was much closer, far better, and far more reliable than our previous provider in this regard. Could usually get to within 25 users of my paid slots before it died. Not a bad choice, though, and certainly know what they're doing and how to manage their hardware.
C(9) --- Warlance has good people. Or, at the very least, a good person seemingly leading the charge. The individual is honest, straightforward, and seems to genuinely care about gamers. When approached about my issues with my Warlance instance, the Warlance rep was straightforward about the limitations of the game itself, and the requirements it needs from your average server with the process being tied to a single thread. But, we'd seen servers run with 200 users, and Warlance pointed us at QIG.
Quantum Impulse Gaming (QIG)
H(10) --- Without a doubt, the QIG servers are the best you can get for Holdfast, hands down. Was able to run 200 users with nearly no lag and almost never a stutter.
C(1) --- The individual running QIG is, in my professional opinion, in it for the money. There have been times when the interface has gone down or things have stopped working (reported by multiple other users, not just yours truly, ask BlackFish if you need another example) where QIG did not provide notices to the users of outages, did not care that you couldn't use your service, didn't provide anyone with a get well date, and blamed the user for not finding an alternative means of contacting QIG (through Steam/Discord or some other shenanigans). I personally could not access my service from the get-go after I purchased it, missing 2 events due to this fact, and when I did a credit charge-back for the service I absolutely could not use, QIG's response was to call it fraudulent and blame me for not contacting them on Steam. I'm not their gaming buddy, and certainly not their friend. Why would I have them on Steam? I'm not impressed with them, nor is anyone else I've spoken with.
My ultimate recommendation from these three? Go with QIG if you absolutely need the best of the best, but I think even now half their servers are not responding and they haven't responded to tickets for days and otherwise don't care. If support and reliability are important to you, however, go with Warlance.