Nearly all opening bids give some information about strength, but here we want to look at bids which show only the strength of the hand, giving little or no information about shape. The most important example here is the Strong 1C opening in systems like Precision. Also, "natural" systems often use 2C as a strength-showing opening, but because of the low frequency of the bid and the fact that all hands opened 2C are two-bid hands, it is something of a special case and does not really define the system in the same way that the 1C opening is fundamental to Precision. We will be more interested in 1-level strength-showing openings.
The two main problems with strength-showing opening bids have already been discussed in earlier posts, but are worth repeating.
First of all, there is a danger of violating the Unbalanced-Hands-Show-Shape principle. On one-bid hands in particular, it may turn out to be impossible to say anything about the shape of the hand if you start with a strength-showing opening bid. But even on two-bid hands, while you will get the chance to say something about shape, it might not be possible to give such a complete description as you would if the opening bid had already begun to show shape information.
The second problem is that it is difficult to give a good definition of the strength of a hand in isolation, since its power depends a lot on how it fits with the other hands at the table. So, information about strength is most useful if the shape is also well defined. For this reason, if a system has a bid which limits strength very precisely, that bid will usually show shape precisely as well - as for example with a natural 1NT opening. A bid which showed a 3-point range of high-card strength without showing anything about shape would be very suspect - the upper limit on strength is almost useless, since hands can become much more powerful than expected if there is a big fit. Limited openings (ie. those where the maximum strength is very restricted) only really make sense if they show something about shape.
Information about the minimum strength is still useful, of course. As was said in "Balanced Hands Show Strength", if our hand has close to the minimum strength permitted for the opening bid that we choose to make, we can say we have "shown" the strength of the hand even if some much stronger hands are opened with the same bid. So one of the main advantages of strength-showing bids is that they satisfy the requirements of the Balanced-Hands-Show-Strength principle, for balanced hands near the minimum end of the range. For unbalanced hands the information about minimum strength is also undoubtedly helpful, but not as much as it would be if we had shown something about shape as well.
Looking specifically now at a "Strong" 1C or 1D opening bid, this typically promises a minimum of about 16 HCP, maybe slightly more or slightly less depending on how aggressive you want the system to be. Obviously, playing a Strong 1C or 1D opening has a huge effect on the other bids in the system as well, but for now we just want to look at the strength-showing bids themselves.
Such bids are unusual in terms of the proportions of one-bid and two-bid hands that they contain. Most 1-level bids are dominated by one-bid hands. But in a strong opening bid there are very few one-bid hands. Certainly, balanced hands of 16-17 HCP fall into this category. But that is about all. An unbalanced hand with this strength looks more like a two-bid hand: it may not be a pure two-bid hand, but the system really treats it as if it is, because having not shown shape with the opening bid, there is a lot of pressure to do so later. And once you start looking at even stronger hands than this, they might not like to take a second bid in competition, but they don't like to pass either. There is certainly a lack of pure one-bid hands. So these strength-showing openings have essentially the opposite problem to the very limited opening bids discussed in a previous post: while 8-12 HCP opening bids wasted space because they hardly include any two-bid hands, strong openings waste space because they hardly include any one-bid hands.
The small number of one-bid hands also gives responder an unusual problem. Suppose that he has to deal with a low-level overcall (somewhere between 1H and 2S, say). When he has a "positive" hand, good enough to force to game opposite opener's known strength, things are generally fairly easy. But more interesting is when he has a slightly weaker hand, a "semi-positive". These are not good enough to force to game immediately, so they have to be bid carefully, not going past the best part-score. Much of the time, it will be best to pass and wait for opener to describe his hand. However, since opener can occasionally have a one-bid hand, passing may result in the overcall being passed out. So responder is forced to act on a semi-positive hand if he wishes to compete for the part-score opposite a one-bid hand.
Now, if opener actually turns out to have extra values, any action from responder effectively commits the partnership to game. So the range of strength for the semi-positive hand types needs to be very narrow - not good enough to force to game immediately, but happy to play in game if opener has any extras. This does not seem to be very efficient: you are using an awful lot of system (the semi-positive responses and their continuations) to cater for a very small number of hands (opener's one-bid hands). This takes away space that could be used for more common hand types. Well-designed systems can use transfers or suchlike to combine the semi-positives and the positives into a single bid, but you still see the problem with semi-positives when responder makes a double, or where there isn't room for transfers, or when opener is prevented from showing his hand by having to cater for responder's possible minimum.
You can contrast this with the multi-way opening bids discussed previously. The equivalent of "semi-positive" hands for Swedish Club are those hands which want to compete opposite a weak NT hand, with the auction 1C : (2D) : 2S being a classic example. This has a much wider range. At the lower end, it only needs to be good enough for game opposite the strong option, so perhaps 6+ HCP. This is almost the same as opposite a strong opening bid. But the upper limit is determined by whether it is good enough to force to game immediately, which is much higher for the multi-way opening. So these semi-positive responses, which use up most of the available space, are much better used after a multi-way opening.
Some Strong Club systems go so far as to make responder's pass forcing over certain overcalls. In a sense this avoids the problem of having a small number of one-bid hands to deal with, by requiring opener to always take a second bid. But of course this takes away one of the main advantages of strength-showing openings, which is that they describe the strength of their minimum hands (particularly balanced hands) without opener having to take a dangerous second bid.
There are also a few systems which remove minimum balanced hands from the strength-showing opening completely, perhaps putting them into a strong NT opening instead. The idea is to increase the purity of the two-bid hands remaining in the strong opening, which works well when you do actually hold one of those hands. But again, you are losing the very thing that strength-showing openings are best at, which is describing the strength of balanced hands at the minimum end of the range, according to the Balanced-Hands-Show-Strength principle.