If you play subtle jazz, a combo will do great. Metal? Not as likely.
I played with very hard hitting, loud drummers and combo amps never seemed to cut through. At least not with any level of good tone. Open back combo amps tend to bleed lots of the bottom end of the tone at higher volumes. That nice fat tone you dialed in sitting by yourself is likely to scream in a bit of pain if you have to play over someone who plays in a more heavy handed rock/metal style.
Reality was for me, in the day, I had an old 70's Peavy Classic combo that I played through a Turbo Overdrive and Wah and it got great tone at all but the highest volumes. The amp had an 'out' that I could plug in to a 4x12 cabinet. It has massive tone and plenty of volume then.
In my unofficial opinion, the watts can be important but 50 is plenty. If you can get a cabinet (even a 2x12), you'll be able to get a good sound at a decent volume. The wattage does affect the volume but at a point anything has too much volume. Higher wattage amps tend to have the ability to overdrive post tubes or push the speakers in desirable ways. So, there are reasons why high watt amps exist but for all practical purposes, a good amp with decent tone and effects will do a fine job without having to overwhelm.
