That is a pretty broad statement.
Not all slugs are created equal, in this thread the favorites run from 1450 FPS (copper solids) to 1900 (Federal Barnes Expanders) as published. Those speeds are however established through 30" barrels. Most real world slug guns will run 100-150 FPS slower than the published velocities when shot through the chronograph.
All of the slugs mentioned have a reputation of being excellent deer slugs and certainly have brought the slug gun much closer to the performance you can obtain from a muzzleloader, especially those slugs with a BC around .210 pushing 1900 FPS.
The problem starts with if you can get your gun to utilize all of that speed and energy. Not all slugs guns will be accurate with the fastest load available, especially when you start to extend the ranges. That is why copper solids shoot so well from almost all slug guns, they are considerably slower and just easier to get to shoot than the top speed and pressure loads from many guns. Federal says there Barnes expanders are CAPABILE of 4" groups at 200 yds, I believe they are, but I don't believe many slugs guns are capable, or at least I haven't seen many that are. I am also not sure that many may know that with a 100yd zero they will be 14" low at 200 yds (That is the 1900 FPS load) or that they could set a 150 yd zero and be around 3.5 high at 80, and 8 low at 200. That same load has a wind drift, 10 MPH 90 degree wind, of 7" at 150 yds and 12" at 200 yds. These are all numbers that are doable under the right conditions, no doubt
A muzzleloader just provides a lot more load adjustment flexibility than you can obtain with a shotgun slug. Instead of buying boxes of multiple brands (been there done that) to see what my shotgun likes, I can just adjust the powder charge slightly, or change the sabot to achieve maximum accuracy and performance.
I like my slug guns, I have an 870 20 ga with a Hastings barrel, a Mossberg bolt slug gun, a 12 and a 20 H&R ultra Slugsters. They shoot great and have all the accuracy I would ever need from a slug gun if all of my hunting was 150 yds and under.
I hunt wide open country and it does offer longer shots if I choose to take them under the right conditions. The slug guns I own with any ammunition available will not offer me the ballistics or accuracy that I can achieve from my muzzleloaders at longer yardages. I started hunting with my Knights as my primary deer guns around 10 years ago just for the added performance they provide if I ever needed it. Three years ago I kind of retired my Knights and went to the Savage muzzleloaders and now it is not even a remotely close call on performance as compared to any slug guns.
Slug guns are great but I hunt primarily with a MZ only because it has the capability to place a shot where I want it to go at 200 yds and beyond if I choose. I really don't feel handicapped or limited by hunting with a MZ at all. I grew up as a bowhunter and still believe in one well placed shot, the time required to reload has never been an issue or required.
chetter - The Federals shoot well from my Mossberg bolt gun. Inside 3" at 100 yds, I only have a 1X4 varible on that gun. With higher mag it could tighten up a little. Good luck
