Once you're familiar with Wordpress, installing & customizing a theme is easy, but some find it tedious.
This is half true
Depending on the theme, there are often simple controls to customise the look/feel. But where it usually gets sticky if you want to go 'off piste' at all. Then it gets very tedious
But this I mean, lets say you have a great looking theme, but it has a white background for your pages, and you want to change it to a dark colour - if there is no button for it built into the theme you are going to have a hard time changing it yourself.
Similarly, if you want to tweak layout of menus or footers etc, then you may find yourself having to edit files manually or installing extra plugins etc.
Wordpress is great, but it does take a bit of practice to get things working well - if you arent having someone build it for you (and therefore paying them to take the pain to meet your requirements) then take your time playing around and install demos/free themes until you find the one you like best - then build your site fresh with that one.