I do think it is easy to create a character that is under-powered in combat by multi-classing with a more balanced set of levels, unless you do it very carefully to ensure that you're getting the best synergy out of it. The problem is that you're significantly delaying the Cleric's spellcasting progressing, which is how they get good combat damage, and you're reducing sneak attack progression, which is how Rogues get good damage progression. Moreover, you have multi-ability dependency, needing both Wisdom to increase save DC for your spells, and Dexterity to improve finesse melee and/or ranged weapon attacks.
The biggest advantage of multi-classing is that it front-loads you with more abilities, improving your versatility and utility powers, both in and out of combat. A single level dip is very common to get armor/shield proficiency, for which Cleric is good. There are sometimes good archetypes to choose from that make a level 3/3 split relatively strong, particularly at mid-levels. For example, the War Priest archetype for Cleric getting bonus attack Wisdom modifier times before resting, and being able to channel divinity to get +10 to attack on a missed attack roll - those both help ensure that an attack hits, helping get that Rogue sneak attack in. Likewise, the Rogue archetypes such as Arcane Trickster for an attack cantrip (True Strike, Booming Blade, or Green-Flame Blade), or Assassin for advantage on initiative (easily underrated, but quite beneficial), and additional damage in the first round of combat.