I understand your frustration with matches that feel extremely inconsistent, but I feel these problems you are describing are not native to Overwatch. The development team have specifically stated they do not take any measurement of time played into measuring your skill or the skill of your allies and opponents in matchmaking.
Furthermore, the game does reasonably work to pit groups of equal size together, not always but it consistently does. However, this does mean solo players do require to play with groups of various sizes. Most of the time though, you will rarely see cases of all solos versus a six-stack in Competitive. Principal Designer Scott Mercer detailed some facts here:
This is no longer true, Heroes of the Storm merged the Hero League Ranked Mode and Team League Ranked Mode into what is now called the Storm League and allows groups of all sizes to play in it.
You are not suspended if you choose to leave early but you still take the loss and the appropriate skill rating adjustment. It’s rough, but there is a reason why this has to happen. Scott Mercer explained this on the old forums:
Again, I understand the frustration with having matches that feel impossible to win, and sadly that is consistent with any online team-based game no matter how strict or loose the rules are. The important thing to remember is to continue to do the best you can no matter what, because when you do play the matches that do matter, it will be your skill and how good you really are that will truly move you up or down the ranked ladder.