It's important to remember that the reason we show shape is in order to find a fit. (Indeed, we need to be able to find out both where the fit is and how good it is, though these things generally come together.) So, the best shape-showing bids are the ones which maximise partner's chances of knowing (or finding out) what our fit is. Some bids which appear to show shape are actually not particularly helpful for finding fits. The most extreme sort of example is if your system has a bid which shows a hand of 5-4-3-1 shape, but does not indicate which suit is which. If your only aim was to reveal the complete shape of the hand then this would be a fine way to start - later bids could reveal the order of the suits - but for finding fits quickly it's absolutely hopeless.
The standard method of bidding unbalanced hands is to start by bidding your longest suit. One obvious reason why this is sensible is that, a priori, this suit is the one most likely to be your side's best fit. However not all systems work this way. An alternative is "canapé" style, where we bid the second-longest suit if holding a two-suited hand. Pure canapé is rather uncommon, but many systems vary between opening the longest and the second-longest suit depending on the hand type.
The obvious problem with canapé is, how are you going to find out whether you have a fit in your longest suit? You can't rely on partner to bid the suit, because your side might have a fit there even if he does not have a very remarkable holding. Essentially, the only way you can discover a fit in your longest suit is if you are able to bid it yourself. And this may not always be possible. Unless you have a pure two-bid hand, you might not be able to take a second bid at all.
But this doesn't necessarily mean that canapé is a bad idea on one-bid hands. Take a hand like this, for example:
Would you rather open this 1H, showing 4+ hearts, or 1C, showing 4+ clubs? There is a lot to be said for the 1H opening. If partner will expect four cards in whatever suit you bid, then the 1H opening is no less descriptive than 1C. And major-suit fits are more important than minor-suit fits. If 1C did not promise four cards (say you were playing a more artificial system) then it would be even more clear that 1H was a better initial description of the hand. The problems, if there are any, come when you try to refine the description of your hand with a rebid: you would like to be able to show both of your long suits. But since this is a pure one-bid hand, you only expect to get one chance, and so it is reasonable to bid on the assumption that you will only get to show one suit. In that case, showing four hearts must be at least as good as showing four clubs.
So, concealing your longest suit works fairly well when you have a pure one-bid hand, provided that the bid you choose to make is still a good description (if you were concealing a five-card major, it would be much worse). The real problems with canapé come when you have a hand which may be worth a second bid, and particularly if it is neither a pure one-bid hand nor a pure two-bid hand, but somewhere in between.
You open this 1H, LHO bids 2S and it is passed back to you. Now you certainly don't want to pass, but what do you do? Well, this would depend on your methods, and there are many different possibilities, but nothing looks particularly attractive. The natural method would be to show the club suit directly, but this gives up on playing in diamonds or defending spades. Alternatively you could double, if this is defined as take-out, but now you haven't revealed your longest suit and so your continuations will be hampered by having to find this out. You also have the difficulty of trying to distinguish this type of hand from one with five hearts, if those are also opened 1H. Indeed, it seems that your initial shape-showing bid is not helping you at all: if you had opened with a strength-showing bid like a Precision 1C, then a take-out double of 2S would describe the shape of this hand equally well as anything you can do after opening 1H.
Similarly, if the opponents instead overcall in diamonds, there is still a fair chance that we have a fit in clubs, but it is dangerous to look for one because if a fit is not found then we have nowhere else to go.
This exposes a basic problem with not showing your longest suit the first time: if you get the chance to take a second bid, then you will not know what to do. On the one hand, you would like to show your longest suit, because this is by far the suit most likely to be a fit: but on the other hand, you can't commit to playing in your longest suit, because there is no guarantee that it will be right.
And so, if you see a pair playing canapé, or other methods involving not showing the longest suit with the opening bid, they will generally do so only with
- pure one-bid hands, where the problems with taking a second bid are not important; and/or
- pure two-bid hands, in cases where it is almost certain that a good description can be given with the second bid.
For example, playing strong NT and 4-card majors, it is reasonable to open 1H with a hand like my first example above. But we would never open 1H on a similar hand with an extra king, because then it might be worth two bids. (In fact it might well be opened 1NT in that case, in order to turn it back into a one-bid hand.) Similarly, canapé often goes together with a strong club, where the natural suit openings show fairly weak hands, which are unlikely to be worth more than one bid. Though here, hands near the top of the range for a limited opening can still be a problem.
The more traditional method of opening the longest suit really comes into its own on these difficult "in-between" hands. If the auction goes in such a way that it is too dangerous to take a second bid, then at least we have already shown the most important feature of our hand. Whereas if we do get another chance, we now have plenty of flexibility in deciding what is the next most important feature to show. Flexibility is the key: having shown the longest suit already, we could later decide to bid a second suit naturally, but we could also choose to double for take-out, or bid no-trumps, or repeat the first suit, or even make a conservative pass. If we had bid some other suit first, then these options might still be available, but they would not all tell partner about our long suit, so we would have failed to give such a good description of the hand in our two bids, as demanded by the One-Bid-Or-Two principle.
Of course there are other good reasons why you might want to open the longest suit. For example, it increases the average length shown by your opening bids. This is part of a much bigger topic on homogeneity that I will come to later.