This section will explaining the benchmark of tenpai with yaku having a bad shape. There is actually very little difference between this part and other situations with bad shape.
Instant riichi with a 2600, 5200 point hand(including riichi)
There is no problem with declaring riichi immediately with this hand at all.
However since you can win in damaten with this hand, there are cases where people stay in damaten. You can choose to stay in damaten, if you realize that the current situation is dangerous or there's a situation that warrants going into betaori for.
Damaten with a 5200 points hand
The majority of modern mahjong players would stay in damaten with a 5200 points hand that has a bad shape. The only exceptions are when the current round of discard is early or when it's obvious that someone is attacking. (If there's already a situation where three exposed melds are made.)
Riichi only increases the points to 8000, and getting ura-dora would be pointless as the effectiveness is diminished. However if it's still early, there's a good chance of getting tsumo, you can try gunning for a ura-dora 1 haneman.
Tenpai with a [really bad shape]
A really bad shape refers to waiting on tiles that no one will discard, like a dora in the middle of a suit. For example:
Or an even more extreme example:
In the first example, waiting for a 5 wan to turn it into a 3 sided wait is a negative way to play. Beginners tend to assume that no one will discard the dora 7 wan. They will continue to wait even if the current situation is impossible to change tiles with. This is an unwise play style. Since you understand that the dora is a tile that will not come out even if you are in damaten, then you should also know that the side effect of riichi that decreases your winning rate is also hugely reduced. It'll at least be a mangan if you declare riichi and tsumo the dora, and it'll be haneman if you get ura-dora 1. Why not just declare riichi?
The second example follows the same reasoning. No one would simply discard a dora in the middle of a suit. Personally, Iwould declare riichi and aim for a tsumo.
True, this type of hand would usually end in ryuukyoku.(This type of dora tanki tenpai has a winning rate of 20 to 30 percent) However, a haneman is guaranteed if tsumo is achieved, and a baiman is possible if there is an ura-dora. It doesn't matter if the winning rate is a little low, right?
The next time will be discussing commonly used techniques during riichi.