I haven't paid that much attention to it, but I did take a closer look tonight as I was constantly running through new maps and playing 10 turns.
Pretty much ALL of the Mercenary ships you can hire that have a Survey capability have a sensor range of... 2. They also tend to have slow speed, and either a very short range or a ludicrously long one.
That makes them almost completely useless, and certainly not worth the money they cost, unless you're one of the few civilizations that doesn't start with a survey ship, and even there, they're much less utility than one would think.
The problem is that they're substantially inferior to the "ordinary" Survey Ship that most civs start with, and really only useful for "clean up" of already explored space (i.e. running around and picking up anomalies that a sensor boat has already found). The slow speed and especially the crippled sensor range makes them completely useless for exploration, which is what you're really interested in from a Survey ship. Their special abilities (besides survey) are nothing that people care about, certainly not in the early game.
Even worse, they're not normally obtainable unless you have a certain resource; since it's unlikely that any civilization (whether they have a Survey Ship already or not) will start obtaining resources before at least turn 30 or more, you're far, far better off simply researching the Interstellar Survey tech and building your own surveyor than buying any one of these.
Let's look at a couple:
The Facade: costs 600 + Promethium + Thulium, moves 4, Sensor Range of 2, Range of 179 (!) and beam/shield of 3/2.
First off, who is ever going to have those two resources for a long time? Building mining starbases is WAY down on the list of priorities for the early game, and you'd have to be lucky enough to find a location that has both (because building 2 starbases to get this is a waste) OR somehow get lucky with planets having them on it. Realistically, you're not getting this ship before turn 50, at the minimum. And then, what's the point? By then, you have Interstellar Survey, and can build a boat that has significantly superior speed and sensor area, and while it might not have any offense/defense, that's not important, because the only time you need either is when surveying the well-defended anomalies, and you need FAR better stats than The Facade has to do so. The Facade's long range is worthless, because it has slow speed and can't be used to explore long distances due to it's low sensor power (slowly blundering around isn't a viable use case). Grade: F
The Path of Rhuin: costs 600 + Promethium, moves 4, Sensor Range of 2, Range of 40 and beam/missile/gun + armor of 1/2/2 +2. Shares range with fleet.
OK, same problem with getting the resource, if it's a bit less pronounced. The incredibly short range makes it's "special ability" COMPLETELY useless - the only use case I can think of is making a fleet of tiny armed ships (no engines, no range modules) and flying them - slowly - over to the heavily defended anomalies. When is that ever going to happen? Otherwise, it has all the problems of The Facade - you can't use it to explore, it's not tough enough to take on defended anomalies, etc. Grade: D
The Prospector: costs 750, moves 10, Sensor Range of 2, Range of 57 and beam + shield of 3 +2.
Finally, something that *might* be useful. For a civ that doesn't start with a Survey Ship, this one makes a passable substitute. The much better speed gives it *some* utility as an explorer, but the horrid Sensor range severely restricts that. At best, this ship is useful for two things: being sent to nearby unexplored star systems to see what's up, and for "cleanup" of anomalies that you REAL sensor boats find inside your empire. It's not cheap, but not excessively expensive either. Grade: B (if you have no Survey Ship to start), C (otherwise)
The Ysengard: costs 1200, moves 5, Sensor Range of 2, Range of 56 and beam/missile/gun + armor of 1/2/2 +2. Gives +25 credits per anomaly.
While there's no resource cost, and thus it's possible to buy early, the cost is prohibitive: 40% of your starting money. That +25 credit bonus is going to take 48 surveyed anomalies to make back, which is easily 100+ turns. So, what is the utility of this power? None, as the up-front cost is so high (when you need the money for tech trading) and by the time you're making the investment back, you've got more than enough income to make the +25 credits irrelevant. Once again, it has pretty much the same issues as the Path to Rhuin - far too expensive early on to be worth buying, bad sensor range to be used to explore, and only modest range and speed. It's useful really only to be sent to relatively nearby star systems, and for anomalies inside the immediate empire. Also, note that there's a text discrepancy: the main Merc tile properly displays that +25 bonus, but if you look down in the text at the lower left where it gives more ship stats, it incorrectly claims +100 instead. Grade: C-
In two years of playing, I've NEVER bought one of these mercenaries. They're that useless.
The ultimate issue is that Surveying isn't really that important early on. Sure, you get some additional money, and the very occasional tech. Which is nice. But, compared to long-range sensors on fast ships, looking for habitable worlds and contacting other races, anomalies are way down on the priority list. Heck, even with a normal Survey Ship, I'm not detouring out of my way much to pick up anomalies until I've completed scouting out the local 40-hex radius of my homeworld. A nice sensor boat (which is buildable no more than 15 turns from the start, using high-priority tech you want to get for other reasons anyway) that you rush buy is FAR more useful than any of these Merc ships.
And you don't want any of them after about turn 50 anyway, as they provide no benefit you can't get elsewhere for less cost.
At a bare minimum, every single one of those ships needs to have a sensor range of 4, and probably 5 or 6 to make them worth their cost, as they can then at least perform as well as the base Survey Ship does. A balance pass on their other stats also should be done, to see how they can be make useful for other things. But the current lack of exploration capability for them is a deal-breaker.