Niagara has a strong reputation for family play and innovative game play, but I think the game has a few faults. Let me set the stage for these by saying that there are a number of reviews on this site that are spot on with their praise for this game. Instead of re-stating those points or all the rules for the game I just want to hit the main points.

1. The game board mechanic does not always work as intended.

The game board is set-up so that a row of plastic disks is placed into a recess in the board and forms a "river." This river is moved by placing new disks at one end and pushing the whole line of disks along.

The problem is that the disks do not always push each other along when a new disk is added. At the "folds" in the board these disks can be pushed under or over the disks in front of them if you aren't paying attention. (You won't be because you'll be watching to see if your canoe falls off the board.) Big deal right? Just fix the problem. AHA, now you will see the problem - YOU will fix the disks. If/When a disks get over/under near the fork in the river, you will have to decide which fork the disk will now travel. Multiply that by the number of times it happens a game and you have the potential for "fixing" the outcome - even unintentionally. If you are playing with children, you know the danger of being the "grown-up" who will be the one who fixes these problems.

The most effective way of reducing these events is to weigh down the board at the "folds." Unfortunately I feel that the board is already too crowded with canoes, paddle cards, and gems for something with enough weight to be placed on the board - especially in games with many players.

Conclusion: People play this game for the river mechanic - if it doesn't work it loses the "river" fun and becomes "movement penalty game mechanic" - possibly influenced by players.

2. The rules are more complicated than they need to be.

The rules do not lend themselves to an easy understanding of the game. I prefer to allow a first-time player skim the rules before trying to explain the game - usually while I set the game up. I would be very reluctant to hand this rulebook over to a new player - it confused me, and I had played the game before!

Conclusion: The game isn't difficult to teach, but you really need to have an experienced player to teach new players - the rulebook isn't a great introduction to the game.

3. The pieces are too similar in color, specifically the gems.

The object of the game is to collect gems from alongside the river. These gems are different colors. During game play, these gems may be mixed together on the gem holding spots and in the player's scoring area.

The blue and purple gems are too similar in color. During set-up, these gems may become intermixed and you may miss it if the wrong colored gem is under a pile of the correct color. Even more of a problem occurs when a gem is placed back on the bank. The worst problem occurs in identifying the gems in the player scoring area, where they may be no blue/purple gems to compare against.

Conclusion: Gems of the wrong color getting picked up at the wrong spot is fixable but can be tedious as long as you notice it in time. If the gem has already been scored, it's almost impossible to go back far enough and replay the game from the spot of the gem mistake.
I am not colorblind, but I would guess that the colors of the gems are especially troublesome.

Overall
Niagara is a good game, but at times I feel that I am "running" the game instead of "playing" the game. When the whole thing is working like it should, it is fun. When I need to "adjust" the board/disks, or to keep clarifying the color of the gems to family members - it can get slightly tedious.

Recommendations:
Watch a game before trying to play - the game teaches itself better than the instructions.
As a new player - play a few games solo (playing as multiple players) to learn the game.
Only share the back page of the rulebook with new players.
A house rule will need to be made about the "over-under disks" to prevent problems between little ones (or big ones I guess.)
You could play with alternate gems of your choice. These "gems" should be different colors AND different shapes.