Dominos are a type of mortise and tenon joint. Their primary limitation is the size of the dominos. If we want to make a mortise and tenon joint in a project, we make it a little smaller than the pieces to be joined. With dominos, you can't do that (without making your own custom piece and linking multiple slots). Using multiple dominos helps but doesn't eliminate this limitation.
All the biscuits I have used have been hardwood, not plywood or particle board. I've never seen anything other than hardwood used, I believe it is beech. They are slightly compressed and swell when they are placed into a joint and glue is added. The biscuits do not add as much strength because they are not as big. They are wide but that isn't the direction you need, the biggest, a #20, is about an inch wide. So you get something like a mortise and tenon but it only goes 1/2 and inch or less into each piece to be joined. And the domino is only about 1/8 wide. The smallest domino is 4mm thick or about 1/6 of an inch. Even the little domino will make mortises about 3/8 thick. Because of the thin aspect of the biscuits, I think of them more like splines than tenons.
Dominos are quick but limited due to their width, not their thickness. With the bigger domino, you can get plenty of length for most projects, the little one is limited to 25mm - about an inch. But other than the limited width selection available, they make mortise and tenon joints just as strong as any other mortise and tenon. They just make the joints faster.
So is a spline joint equivalent to a mortise and tenon? Obviously not at least for most purposes. Does a spline add strength? Of course it does - but if you use one in a leg to apron joint or other improper application you will be disappointed. But all joints have to be properly applied.