[Suicide Junkie]

The reason I don't like QNP without engine limits is that you can have ships that fly around at 20 sectors per turn.

I have a ship or two like that in my game at home. They are DS, with basic C&C, a temporal sensor and 23 Gravitic drives.

Even with 15 movement points - that's an extra planet or two destroyed in some systems in one turn.

Perhaps new, undefended colonies, but not any world with a half-assed (even AI-generated) defense in place.
You have to sacrifice all of your defenses, and almost all of your weaponry in order to get those really high speeds, so a platform or a handful of sats will swat your ships like bugs.
Sure, if you send enough ships, you can do plenty of damage, but you are spending a huge pile of resources to maintain all those engines.

Anything more than 10-12 movement points is too much.

I totally agree. The warships I design move only 5 MP at the very beginning, increasing to 8 or 9 by the endgame as engines get more efficient.

The key point to the QNP system in P&N is that the limits are not arbitrary. They are economical. Top of the line engines can cost 1000 rads each.

You can make all of your ships move 20 sectors per turn, but they will be worth a king's ransom to build, and be worth squat in combat.
Special forces ships, spyships, and medical ships are potentially worth giving 20 MP. Their key abilities are concentrated into a few KT, and can afford the space.

Transports, ships of the line, and other vehicles are decidedly not worth the cost.
It is usually better to have a transport take twice as much cargo while taking twice as long to get there, and costing half as much.

The point is that the player gets to decide. I do not limit the number of engines because I hate the reason of:
"Because I said so"