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.
5E really rewards taking only a few levels (sometimes only one) of the extra class. The Sorlock, for example, is almost all Sorcerer.
I miss 2nd edition dual-classing. I really did prefer the earlier editions, even for tabletop play.
