Lossy codecs suck at gapless playback. Some have support via vendor-specific extensions, but it’s hit or miss whether a particular player will support it. With FLAC you can always be 100% sure that your album is gapless.
E.g. create a full-scale sine wave, split it into two files, then convert them to "gapless" Opus. Now open the files in Audacity and you'll see that there's a small amount of ringing at the boundary, so it is not truly gapless.
If you try the same with AAC using Apple's gapless metadata (i.e. iTunSMPB) you'll find that the boundary is perfectly continuous.