Ken Burnside said it best, "Friends don't let friends use reaction less drives in their universes."
With all due respect that is good advice but a little too specific and at the same time absolute. It is too specific in that there are many, many technologies that can ruin a story or adventure. Star Trek's transporter is one of the best known. It can just pul your characters out as soon as they're in over their heads, while faux Roman soldiers empty their submachine guns through the sparkles.
The problem with a reactionless drive (from a story point, not a physics point) is that with it any vessel becomes an engine of planetary destruction. Just get far enough away from your target aim it, turn on the drive and get off. In a few months (or less) you have a missile heading at your Evil Overlord at nearly speed of light. Can't even detect that darn former trash hauling scow until the ka-boom.
In the case of the transporter they came up with reasons it couldn't be used, environmental factors, jamming, technical problems, the Captain being an idiot and getting his communicator taken away. In the case of a 'reactionless drive' all you need is a delta vee, a fixed limit to acceleration.
First let's define reactionless drive. A reactionless drive usually means propulsion that moves something without an equal and opposite force. A reaction drive moves stuff by squirting mass out one end very quickly. There are a lot of different reaction drives, real and theoretical, and people are working all the time to make them better. For now in game terms using a reaction drive to get places in a game means that maybe your character's descendants get to embark on the adventure you drew up. They all kind of suck unless we're talking really high energy drives using fusion or antimatter and do you really want your average players characters to have a fusion torch or antimatter? I used to worry when my crew started stocking up on chainsaws in Traveller.
So reaction drives as we understand them miiiiiiight have drawbacks too.
It might be easier to whip up some speed limits for our reactionless drive. What's the big deal? You probably already have an FTL drive. There's some fancy handwaving right there. FTLs generally have some limits. No boarding your ship and engaging the FTL from the spaceport tarmac to appear at your destination's doorstep instantly. There are a couple ways to make a reactionless drive more reasonable. Some of them may even have a basis in fact!
1) It's not really reactionless. It uses gravity or another exotic force to push or pull on one or more bodies relatively nearby. Distance from a massive body will diminish this force and acceleration will fall off (similar to a solar sail that uses light from the sun or magnetic sails that use the solar wind). At a certain point you'd better get out and push. There's a certain prestige that comes from using an entire planet or star for reaction mass.
2) Increasing your ship's speed means increasing its kinetic energy. This increases by the square of the velocity (which is pretty fast!). At some point you're going to run into diminishing returns and a working cap on your speed (relative to the system's star or homeward or whatever.)
3) Technical problems. The drive causes a build up of some something equivalent to static (or really static). Every few hundred kilometers per second of acceleration you use it for requires you to pass through a planet's atmosphere to discharge it. Note this generally. means slowing down to a few kilometers per second relative to the planet you are dumping on. Note also just letting the stuff build up will have very bad effects on your ship long before you brush against relativity. They used a similar approach in the Renegade Legion setting. FTL built up radiation that had to be discharged in normal space eventually. If there was no normal space outside your hull, the normal space inside the hull would do (most crews had a problem with this.)
4) Fuel/Power Hog. having a ship with virtually unlimited atomic or fusion power can stretch credibility at times. Most settings assume we will have relatively cheap and portable fusion reactors and that player character types will be trusted with one for their ship. Refer to my previous comment about the chainsaws please. What if fusion power was reserved for stations and planetoid bases? What is to say a reactionless drive ship can 't be powered by chemical fuel cells (FireFly's Serenity was at least they mention fuel cells) and a ship's fuel load determines its delta vee before you refuel? This turns the tyranny of the rockets hip to more of a constitutional monarchy.
5) Pulling limits out of your @$$. I did this with my ghost drive. That drive used non-biological entities (NBEs) to propel ships. You needed a containment unit for the spooks and had to supply energy. But if you used them too long before a cool down they grew too powerful and could break your containment at which point Scooby-Doo and Mystery Inc. would be no damn good at all. Come up with something cool. Maybe the drive causes insanity or STDs if you leave it on too long without a cool down. If you're going to handwave laws of nature go for broke.
Regardless how hard you like your SF there are some rules conscientious writers follow: they set up the rules and capabilities of their technology, the way the world works first. Then they write stories using those rules. If they give a reason for a ship's top speed to be 1000 kilometers per second then exceeding that speed will be a major plot point (a slingshot maneuver around a neutron star or hacking a linear accelerator). It won't be forgotten as soon as it becomes inconvenient and explained with double talk.