"Generally you simply restricting yourself on gaining higher level spells and/or abilities of your original class. Which are significantly more powerful than anything of lower level. No abilities synergy would compensate that. In many cases something might sound cool. But as you hard test it - you would quickly get it's cool only in theory, but crap on practice. Solo-classed party member always outbest any multiclassed character in 5e."
This is largely true. There are a few classes like ranger where their very top level abilities weren't all that strong, so picking up some versatility might be worth it. I myself am a huge cantrip junkie because I find them very useful out of combat for roleplay and problem solving. Also short games where you aren't expecting to level up could find uses for niche combos that work well at that specific stage of the game. For the most part though multiclassing is done in 5e for fun and flavor, or becoming a jack-of-all because you don't like relying on your party rather than actually making you a monster statistically like in 3.5.
