Again, traditionally opinions range from negative to neutral, so yes that would be the suggestion. In horary, it is generally seen as a major negative. Natally, some view it as a minor debility while others view it in a more dire light.
The nature of conjunctions are that they are unions, so no planet will function on its own, by definition.
As to your example, there would be a mutual colouring of each planet involved. I haven't seen planets being totally cancelled by their conjunction to the Sun, but their is a certain sense of subjectivity that I've seen manifested with the planet conjunct the Sun that in some cases would be a limitation on the part of the native. Sometimes there is wonkiness to it's manifestation as well. Look to the house ruled by the combusted planet to see how the planet fares as a house ruler. Then, the planet will have bearing on whatever house the sign Leo happen to fall in. And the nature of the conjunction will manifest differently according to other aspects of the chart.
Of examples I can think of off the top of my head-
- Native with Sun-Venus Virgo is highly sexed and had unconventional sex partners. She is liable to cheat on her partner due to her sexual proclivities.
- Native with Sun- Venus Virgo is overweight, a sychophant, craved social approval and would do very manipulative and unscrupulous things in order to gain status.
- Sun - Mercury Virgo is seen as slower than he actually is, as he has a very subjective and idiosyncratic way of communicating. He is by no means a dummy, but he is limited in his reception by the way he communicates
- Sun-Mercury Gemini (Cazimi too) has no ability to see beyond her own viewpoints and believes that her way of interpreting the world is the right and true way. She reports that her friends see her as "not as smart as them" and she often feels misunderstood.
- Assemblies (older term for a bunch of planets in the same sign) with the Sun involved also doesn't seem to obliterate the expression of the planets involved, on the contrary in fact. But those are more complex to delineate because of the interplay between so many planets.
To sum up, the planet isn't obliterated totally, but while I don't see struggle, there is a certain level of overemphasis that the Sun conjunction might bring to the planet that may cause an imbalance. Balance and golden means are prized in the tradition. Hopefully this provided clarity.